But the crisis over nuclear weapons didn't come out of nowhere. Here's what you need to know about North Korea's long-standing quest to acquire nuclear weapons:

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Where does North Korea's nuclear weapons program stand now?

North Korea first tested a nuclear weapon in 2006, and it has conducted four tests since then. The most recent, in September, produced a blast that was twice as large as the one that destroyed Hiroshima, Japan, in 1945.

A recent U.S. intelligence estimate suggests that the country may have as many as 60 nuclear weapons, including some that are miniaturized — meaning it might be possible to fit them on a missile. North Korea has also been testing intercontinental ballistic missiles, including one that analysts think could theoretically reach the U.S. mainland.

There remain a number of big questions about the reliability of such missiles and other practical issues, such as their ability to reenter the atmosphere intact. But many experts say that Pyongyang has now crossed a major threshold in its weapons program.

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When did North Korea's weapons program start?

North Korea's attempts to acquire a nuclear weapon go back to the 1960s, when North Korean leader Kim Il Sung asked the Soviet Union, Pyongyang's most important ally at the time and already a major nuclear power, for help with its nuclear program. Moscow rejected Kim's plans and privately pushed North Korea to join the nuclear Non-Proliferation Treaty, an international agreement designed to limit the spread of nuclear weapons.

In the 1990s, however, tension grew, with U.S. officials saying that the country was covertly continuing its weapons program and with disagreements over International Atomic Energy Agency inspections of weapons sites. Over the years, a number of agreements broke down. In 2003, North Korea claimed that it had pulled out of the nuclear pact.

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Under the leadership of Kim Jong Il, Kim Il Sung's son, who ruled from 1994 to 2011, North Korea's nuclear program grew relatively slowly. However, since Kim Jong Un took over in 2011, the country has diverted considerable resources to the program, making a number of major advances in both its nuclear weapons and missiles programs.

Why does North Korea want nuclear weapons?

North Korea's initial drive to have nuclear weapons wasn't unusual in the Cold War era. Many other nations, including South Korea, were seeking nuclear weapons around that time. Kim Il Sung also feared that the United States and South Korea were gearing up to wage a war to unite the Korean Peninsula.

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The end of the Cold War presented an existential threat for North Korea. The Soviet Union, its most important geopolitical ally, had collapsed and was being replaced by a weaker, pro-Western Russia. In anticipation of its upcoming isolation, Pyongyang began investing in its nuclear program, hiring former Soviet engineers who could teach the North Koreans. The nuclear program gave North Korea international clout far bigger than its stagnating economy and diplomatic isolation would normally afford.

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Many analysts now say that Kim views nuclear weapons as a deterrent to a foreign military intervention or regime change. Others argue that the eventual aim for Pyongyang is to reunite the Korean Peninsula under North Korean rule, as it tried with a 1950 invasion.

Why can't the U.S. or another country take out North Korea's weapons program with a military strike?

North Korean state media has repeatedly suggested that it would view the smallest military provocation as an act of war. If that is correct, the results could be disastrous. Putting aside nuclear weapons for a moment, there's little doubt that a full-fledged war against North Korea would result in an enormous loss of life.

Seoul is just 30 miles from the Korean Demilitarized Zone. The South Korean capital region, with its population of about 25 million, is within reach of North Korea's artillery. If North Korea decided to use these weapons, they could cause huge damage in a short amount of time. One study from 2012 estimated that 64,000 people could be killed by North Korean artillery in the first day, but other estimates are far higher. Destroying this artillery would be tricky and time-consuming.

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Even if the United States and its allies were able to quickly take out Pyongyang's military command, it is likely that the damage caused to the country and the ensuing collapse of the North Korean state would create an enormous humanitarian crisis. Because of this, the United States and its allies have largely avoided targeted strikes against North Korean weapons out of fear of prompting a wider war.

What other policies are there?

The United Nations placed sanctions on North Korea after its 2006 nuclear test. The United States has also imposed its own sanctions on Pyongyang, as have the European Union, South Korea and Japan. The most recent U.N. sanctions against North Korea are particularly wide-ranging and aim to cut off a third of North Korea's estimated $3 billion annual revenue from exports.

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The extent to which sanctions can compel North Korea to give up or halt its nuclear weapons program is widely debated. There is also considerable doubt about whether countries such as China and Russia are willing to fully implement U.N. sanctions, dimming their effectiveness considerably.

For many, the ultimate hope with both sanctions and threats of military action is to bring Pyongyang to the negotiating table. The new South Korean president proposed new talks last month.

Would Kim Jong Un use his nuclear weapons?

The brutality of the North Korean regime is notorious. The United Nations says that Kim Jong Un presides over a vast network of political prisons based on the Soviet Union's infamous gulag system.

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But there is little sign that Kim is suicidal. The North Korean regime is probably well aware that it could not survive a nuclear war with the United States — it is outmatched in terms of both nuclear and conventional weapons. Instead, North Korea's weapons seem to be a bargaining chip.

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The U.S. government is well are of this. Despite his tough words this week, Trump has also voiced support for sanctions and talks. But with rhetoric heating up, a miscalculation by either side remains an alarming possibility.