Hong Kong (CNN) On the Korean peninsula, the jittery season is here.

Joint military exercises conducted by South Korea and the United States each spring start this week.

The drills, involving thousands of troops and state of the art military hardware, don't go down well with North Korea. It fired two short-range ballistic missiles into the East Sea, also known as the Sea of Japan, on Monday morning after slamming the exercises as "dangerous nuclear war drills for invading the DPRK."

This undated file picture released by KCNA on January 5, 2009 shows an artillery unit conducting a drill at an undisclosed location in North Korea.

This undated file picture released by KCNA on January 5, 2009 shows an artillery unit conducting a drill at an undisclosed location in North Korea.

A missile is displayed during a military parade to mark 100 years since the birth of North Korea's founder Kim Il-Sung in Pyongyang on April 15, 2012.

A missile is displayed during a military parade to mark 100 years since the birth of North Korea's founder Kim Il-Sung in Pyongyang on April 15, 2012.

This picture from KCNA on December 12, 2012 shows the North Korean rocket Unha-3, carrying the satellite Kwangmyongsong-3, lifting off from the launching pad in North Korea.

This picture from KCNA on December 12, 2012 shows the North Korean rocket Unha-3, carrying the satellite Kwangmyongsong-3, lifting off from the launching pad in North Korea.

North Korea conducted its third nuclear test on February 12, 2013 -- the first under Kim Jong Un. The nuclear test site and water cooling plant are seen in this satellite image.

North Korea conducted its third nuclear test on February 12, 2013 -- the first under Kim Jong Un. The nuclear test site and water cooling plant are seen in this satellite image.

North Korea test-fired a new "ultra-precision" intelligent rocket to be deployed across its navy, the state-run Korean Central News Agency said on February 7, 2015.

On May 9, 2015, North Korea state media reports the country has successfully fired a "cutting-edge" anti-ship missile from a submarine.

On January 6, North Korea claims to have successfully tested a hydrogen bomb . Seismic waves indicate an "artificial earthquake" near Punggye-ri, North Korea's main nuclear testing site.

On February 9, 2016 South Korea's Defense Ministry releases images of debris believed to be a part of North Korean rocket, which was launched on February 7. Pyongyang said it had successfully launched Earth observation satellite Kwangmyongsong-4 into orbit.

Images published by North Korean state media purport to show a submarine-launched ballistic missile (SLBM) off the eastern coast of the Korean peninsula on Saturday, April 23, 2016. Five days later, South Korea claims the North launched two more missiles on April 28 that failed.

A new type of intermediate-range missile, the Pukguksong-2, was launched by North Korea on February 12. It was their first launch since US President Donald Trump took office.

A new type of intermediate-range missile, the Pukguksong-2, was launched by North Korea on February 12. It was their first launch since US President Donald Trump took office.

"Each year, Pyongyang complains and demands a stop to these annual exercises, which it claims to be offensive in nature," said Sung-Yoon Lee, a professor of Korean studies at Tufts University. "North Korea has complained vociferously at least over the past quarter century."

So what's at stake? We take a closer look at the drills and the tensions that surround them.

Why does North Korea get so worked up?

The United States and South Korea stress that the exercises, named Foal Eagle and Key Reserve, are defensive and non-provocative in nature. The North Korean regime, however, doesn't see it that way, and its state media has characterized the drills as rehearsals for an attack.

Leader Kim Jong Un called for full combat readiness and oversaw military facilities, according to KCNA.

"The North Koreans, being paranoid in their own way, have always had this concern: 'If there is going to be an invasion, this would be the time,'" said Philip Yun, executive director of the Ploughshares Fund, a group that advocates nuclear disarmament. "But that's not the intent on the U.S.-South Korean side."

In March 2013, the North Korean military went as far as claiming that the United States was carrying out the drills with the aim "to mount a preemptive nuclear strike together with its South Korean puppet forces."

Playing up the threat also helps the North Korean leadership's propaganda efforts to control the population of the isolated nation, according to Yun.

As well as providing practice for the forces involved, the exercises send a message that the United States "would defend South Korea in the case of a North Korean invasion," said Tong Kim, a visiting scholar at the U.S.-Korea Institute, part of Johns Hopkins University.

How does North Korea respond to the drills?

Usually, with a lot of angry rhetoric and a series of weapons tests.

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Near the start of the year, North Korea typically demands the cancellation of the exercises. Threats of doom and the firing of short-range rockets or missiles into the sea tend to become routine as the military exercises approach.

That was underscored this year on February 8 when North Korea fired five short-range missiles into the East Sea, according to the South Korean Defense Ministry. The barrage into the waters, also known as the Sea of Japan, came just a day after the North announced it had successfully tested a "cutting-edge" anti-ship missile.

The U.S. and South Korean militaries announced Tuesday that the exercises will run from March 2 to April 24. Some furious statements from North Korea followed, including a statement on KCNA that the drills were a smokscreen to "to cover up their surprise invasion of the north."

Pyongyang's outrage ignores the fact that its forces carry out their own drills each winter that analysts view as offensively minded.

Have the U.S. and South Korea ever suspended their drills?

Yes, amid nuclear negotiations with Pyongyang in early and mid-1990s, Washington held off on the drills several times.

"There is precedent, we have done this before," said Yun. But he qualified that "circumstances have changed significantly since that period of time."

Photos: Kim Jong Un and North Korea's military Photos: Kim Jong Un and North Korea's military North Korean leader Kim Jong Un meets with North Korea's first female fighter jet pilots in this undated photo released by the country's state media on Monday, June 22. He called the women "heroes of Korea" and "flowers of the sky." Hide Caption 1 of 55 Photos: Kim Jong Un and North Korea's military Kim stands on the snow-covered top of Mount Paektu in North Korea in a photo taken by North Korean newspaper Rodong Sinmun on April 18 and released the next day by South Korean news agency Yonhap. Kim scaled the country's highest mountain, North Korean state-run media reported, arriving at the summit to tell soldiers that the hike provides mental energy more powerful than nuclear weapons. Hide Caption 2 of 55 Photos: Kim Jong Un and North Korea's military Kim Jong Un, center, poses with soldiers on the snow-covered top of Mount Paektu in an April 18 photo released by South Korean news agency Yonhap. Hide Caption 3 of 55 Photos: Kim Jong Un and North Korea's military Kim visits the Kumsusan Palace of the Sun in Pyongyang, North Korea, on April 15 to celebrate the 103rd birth anniversary of his grandfather, North Korean founder Kim Il Sung. Hide Caption 4 of 55 Photos: Kim Jong Un and North Korea's military Kim inspects a drill for seizing an island at an undisclosed location in North Korea in an undated picture released by North Korea's official Korean Central News Agency on February 21. Hide Caption 5 of 55 Photos: Kim Jong Un and North Korea's military Kim speaks during a meeting of the Political Bureau of the Central Committee of the Workers' Party of Korea in Pyongyang, North Korea, in this photo released February 19 by the state-run Korean Central News Agency. Hide Caption 6 of 55 Photos: Kim Jong Un and North Korea's military A picture released by the North Korean Central News Agency shows North Korean leader Kim Jong Un appearing without his cane at an event with military commanders in Pyongyang on Tuesday, November 4. Kim, who recently disappeared from public view for about six weeks, had a cyst removed from his right ankle, a lawmaker told CNN. Hide Caption 7 of 55 Photos: Kim Jong Un and North Korea's military Kim is seen walking with a cane in this image released Thursday, October 30, by the state-run Korean Central News Agency. Hide Caption 8 of 55 Photos: Kim Jong Un and North Korea's military Kim sits in the pilot's seat of a fighter jet during the inspection. Hide Caption 9 of 55 Photos: Kim Jong Un and North Korea's military This undated photo, released Tuesday, October 14, by the KCNA, shows Kim inspecting a housing complex in Pyongyang, North Korea. International speculation about Kim went into overdrive after he failed to attend events on Friday, October 10, the 65th anniversary of the Workers' Party. He hadn't been seen in public since he reportedly attended a concert with his wife on September 3. Hide Caption 10 of 55 Photos: Kim Jong Un and North Korea's military A picture released by the KCNA shows Kim and his wife watching a performance by the Moranbong Band on Wednesday, September 3, in Pyongyang. Hide Caption 11 of 55 Photos: Kim Jong Un and North Korea's military Kim tours a front-line military unit in this image released Wednesday, July 16, by the KCNA. Hide Caption 12 of 55 Photos: Kim Jong Un and North Korea's military Kim poses for a photo as he oversees a tactical rocket-firing drill in June. Hide Caption 13 of 55 Photos: Kim Jong Un and North Korea's military Kim watches a tactical rocket-firing drill in June. Hide Caption 14 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier patrols the bank of the Yalu River, which separates the North Korean town of Sinuiju from the Chinese border town of Dandong, on Saturday, April 26. Hide Caption 15 of 55 Photos: Kim Jong Un and North Korea's military In this photo released Thursday, April 24, by the Korean Central News Agency, Kim smiles with female soldiers after inspecting a rocket-launching drill at an undisclosed location. Hide Caption 16 of 55 Photos: Kim Jong Un and North Korea's military A picture released Tuesday, March 18, by the KCNA shows Kim attending a shooting practice at a military academy in Pyongyang. Hide Caption 17 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier uses binoculars on Thursday, February 6, to look at South Korea from the border village of Panmunjom, which has separated the two Koreas since the Korean War. Hide Caption 18 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier kicks a pole along the banks of the Yalu River on Tuesday, February 4. Hide Caption 19 of 55 Photos: Kim Jong Un and North Korea's military A photo released by the KCNA on Thursday, January 23, shows the North Korean leader inspecting an army unit during a winter drill. Hide Caption 20 of 55 Photos: Kim Jong Un and North Korea's military Kim inspects the command of an army unit in this undated photo released Sunday, January 12, by the KCNA. Hide Caption 21 of 55 Photos: Kim Jong Un and North Korea's military Kim visits an army unit in this undated photo. Hide Caption 22 of 55 Photos: Kim Jong Un and North Korea's military Kim inspects a military factory in this undated picture released by the KCNA in May 2013. Hide Caption 23 of 55 Photos: Kim Jong Un and North Korea's military Kim visits the Ministry of People's Security in 2013 as part of the country's May Day celebrations. Hide Caption 24 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier, near Sinuiju, gestures to stop photographers from taking photos in April 2013. Hide Caption 25 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers patrol near the Yalu River in April 2013. Hide Caption 26 of 55 Photos: Kim Jong Un and North Korea's military Kim is briefed by his generals in this undated photo. On the wall is a map titled "Plan for the strategic forces to target mainland U.S." Hide Caption 27 of 55 Photos: Kim Jong Un and North Korea's military Kim works during a briefing in this undated photo. Hide Caption 28 of 55 Photos: Kim Jong Un and North Korea's military In this KCNA photo, Kim inspects naval drills at an undisclosed location on North Korea's east coast in March 2013. Hide Caption 29 of 55 Photos: Kim Jong Un and North Korea's military Kim, with North Korean soldiers, makes his way to an observation post in March 2013. Hide Caption 30 of 55 Photos: Kim Jong Un and North Korea's military Kim uses a pair of binoculars to look south from the Jangjae Islet Defense Detachment, near South Korea's Taeyonphyong Island, in March 2013. Hide Caption 31 of 55 Photos: Kim Jong Un and North Korea's military Kim is greeted by a soldier's family as he inspects the Jangjae Islet Defense Detachment in March 2013. Hide Caption 32 of 55 Photos: Kim Jong Un and North Korea's military Kim is surrounded by soldiers during a visit to the Mu Islet Hero Defense Detachment, also near Taeyonphyong Island, in March 2013. Hide Caption 33 of 55 Photos: Kim Jong Un and North Korea's military Kim arrives at Jangjae Islet by boat to meet with soldiers of the Jangjae Islet Defense Detachment in March 2013. Hide Caption 34 of 55 Photos: Kim Jong Un and North Korea's military Soldiers in the North Korean army train at an undisclosed location in March 2013. Hide Caption 35 of 55 Photos: Kim Jong Un and North Korea's military In a photo released by the official North Korean news agency in December 2012, Kim celebrates a rocket's launch with staff from the satellite control center in Pyongyang. Hide Caption 36 of 55 Photos: Kim Jong Un and North Korea's military Kim, center, poses in this undated picture released by North Korea's official news agency in November 2012. Hide Caption 37 of 55 Photos: Kim Jong Un and North Korea's military Kim visits the Rungna People's Pleasure Ground, under construction in Pyongyang, in a photo released in July 2012 by the KCNA. Hide Caption 38 of 55 Photos: Kim Jong Un and North Korea's military A crowd watches as statues of North Korean founder Kim Il Sung and his son Kim Jong Il are unveiled during a ceremony in Pyongyang in April 2012. Hide Caption 39 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier stands guard in front of an UNHA III rocket at the Tangachai-ri Space Center in April 2012. Hide Caption 40 of 55 Photos: Kim Jong Un and North Korea's military In April 2012, Pyongyang launched a long-range rocket that broke apart and fell into the sea. Here, the UNHA III rocket is pictured on its launch pad in Tang Chung Ri, North Korea. Hide Caption 41 of 55 Photos: Kim Jong Un and North Korea's military – A closer look at the UNHA III rocket on its launch pad in Tang Chung Ri, North Korea. Hide Caption 42 of 55 Photos: Kim Jong Un and North Korea's military A military vehicle participates in a parade in Pyongyang in April 2012. Hide Caption 43 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers relax at the end of an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012. Hide Caption 44 of 55 Photos: Kim Jong Un and North Korea's military Kim Jong Un applauds as he watches a military parade in Pyongyang in April 2012. Hide Caption 45 of 55 Photos: Kim Jong Un and North Korea's military A North Korean soldier stands on a balcony in Pyongyang in April 2012. Hide Caption 46 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers march during a military parade in Pyongyang in April 2012. Hide Caption 47 of 55 Photos: Kim Jong Un and North Korea's military Soldiers board a bus outside a theater in Pyongyang in April 2012. Hide Caption 48 of 55 Photos: Kim Jong Un and North Korea's military North Korean performers sit below a screen showing images of leader Kim Jong Un in Pyongyang in April 2012. Hide Caption 49 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers salute during a military parade in Pyongyang in April 2012. Hide Caption 50 of 55 Photos: Kim Jong Un and North Korea's military North Korean soldiers listen to a speech during an official ceremony attended by leader Kim Jong Un at a stadium in Pyongyang in April 2012. Hide Caption 51 of 55 Photos: Kim Jong Un and North Korea's military Members of a North Korean military band gather following an official ceremony at the Kim Il Sung stadium in Pyongyang in April 2012. Hide Caption 52 of 55 Photos: Kim Jong Un and North Korea's military North Korean military personnel watch a performance in Pyongyang in April 2012. Hide Caption 53 of 55 Photos: Kim Jong Un and North Korea's military A North Korean controller is seen along the railway line between the Pyongyang and North Pyongan provinces in April 2012. Hide Caption 54 of 55 Photos: Kim Jong Un and North Korea's military A North Korean military honor guard stands at attention at Pyongyang's airport in May 2001. Hide Caption 55 of 55

North Korea has determinedly pressed on with its nuclear weapons program, thumbing its nose at the international outcry. It has carried out a series of underground tests and launching long-range rockets that could be used as intercontinental missiles.

In January, Pyongyang suggested it would halt nuclear tests if the United States canceled the joint drills, drawing a sharp response from Marie Harf, a U.S. State Department spokeswoman.

"The offer, as I understand it, which we see as an implicit threat, is for the U.S. to stop doing something that is routine, that is transparent, that is defensive in nature, and that is annual ... in exchange for the North Koreans not doing something that is prohibited under multiple U.N. Security Council resolutions and that they are not supposed to be doing," Harf said. "That's really a false choice here."

Why did tensions increase so much in spring 2013?

The tone was set by North Korea's long-range rocket test in December 2012, followed by its third nuclear test two months later.

The United Nations responded with sanctions, and Pyongyang continued to ramp up its threats of nuclear war against South Korea and the United States.

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One North Korean government website even uploaded a YouTube video showing an imaginary missile attack on Washington.

The U.S. decision to fly B-2 stealth bombers , which are capable of carrying nuclear weapons, over the region only served to further antagonize North Korea amid the annual military drills.

"That was a really bad escalation of the tensions in the Korean peninsula," Tong Kim said of the period.

But Pyongyang's decisions to carry out the rocket launch and nuclear test were most likely carefully timed, according to Yun, who was part of U.S. teams that negotiated with North Korea under former President Bill Clinton.

"They game everything out. They don't do things off the cuff for the most part," he said of the North Koreans. "If they're going to do something very provocative, they have an extensive decision tree laying out many options."

The moves appeared to be aimed at advancing North Korea's technology and making Kim Jong Un, still a relatively new leader, look strong inside the country, Yun said. They also coincided with political transitions in South Korea, China and Japan.

Could something similar happen again this year?

Predicting the secretive North Korean regime's next move is a notoriously difficult game.

Last spring, its behavior was less provocative than in 2013, although it did fire off a series of weapons that included medium-range ballistic missiles.

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Tong Kim said that he didn't expect a new nuclear test or long-range missile launch this year, but that short-range missile firings could be a possibility.

An escalation of rhetoric around the military exercises is also likely.

Lee was less optimistic about the near future, saying that, if a major provocation takes place, "no one should be surprised."

"North Korea has shown itself to be calculating and strategically minded," he said. "There's a political need to raise the stakes, there's probably a technical need to test a long-range missile or a nuclear test -- it's been a couple of years."

Yun said he was concerned that a miscalculation in an already sensitive area, like the disputed waters off the Korean peninsula's west coast, could lead to a rapid escalation of tensions.

"There are a lot of things that can happen with a significant number of military assets around and on the peninsula," he said.

Is Kim Jong Un's North Korea different from his father's time?

Kim Jong Un is "similar in action but stronger in rhetoric" than his father, Tong Kim said. "Except that North Korea under Kim Jong Un has newer and more formidable weapons."

Some of the techniques seen under Kim Jong Un certainly recall those employed during his father's rule.

During the tensions in early 2013, North Korea declared that the armistice agreement that halted the Korean War in 1953 was no longer valid.

The announcement sounded unsettling, but North Korea had already said in 2009 that its military was no longer bound by the armistice because South Korea was joining a U.S.-led anti-proliferation plan.

In 2013, the North also tried using the silent treatment, cutting off a military hotline with the South. That was similar to an approach it had adopted in 2009 when it stopped responding to calls after the military exercises started.

But during 14 years of Kim Jong Il's rule, the United States and South Korea "had a track record of what North Korea would do and a sense of what to expect," Yun said. "Kim Jong Un was new, you didn't know how far he would go, which added to the uncertainty."