President Donald Trump's decision to stop "provocative" war games between the US and South Korea after his historic summit with North Korea stunned many observers.

Key points: War games are joint military exercises designed to train and prepare soldiers for real combat

War games are joint military exercises designed to train and prepare soldiers for real combat The US-South Korea exercises are usually conducted every spring and summer

The US-South Korea exercises are usually conducted every spring and summer North Korea has long considered them to be preparations for an invasion

US Republican Senator Lindsey Graham slammed Mr Trump's cost reasoning as "ridiculous", while a former South Korean military general said he was "speechless" at the decision.

The suspension of war games has long been requested by Pyongyang, and has created tensions on the Korean Peninsula. But now, North Korea may finally get its wish.

But what are war games? Why are they such a point of contention and are they actually an expensive threat to Pyongyang?

What are war games?

The US and South Korean military typically hold major joint exercises twice a year. ( Yonhap via AP: Kim Jun-bum, file )

War games are joint military exercises designed to test, train and prepare soldiers for real combat, by simulating real battle situations and scenarios.

Military personnel conduct mock search and rescue missions, amphibious beach landings and coordinated bombing scenarios.

Computer-simulated exercises are also conducted, with the goal of ironing-out decision-making and planning processes.

Sorry, this video has expired Talisman Sabre military exercise kicks off with biggest amphibious landing since WWII

Michael Shoebridge, the director of defence and strategy at the Australian Strategic Policy Institute, said it was a matter of practice makes perfect.

"It's like an orchestral concert. Practice matters for those things to get it right on the night, well it's exactly the same with militaries."

Militaries across the world hold war games with their allies, including Australia. Our biggest exercise is operation Talisman Sabre, held with the US military every two years.

How often are they held in South Korea?

US and South Korea hold a joint live-fire military exercise at a training field near the demilitarized zone. ( Reuters: Kim Hong-Ji )

The US and South Korea's major joint military exercises are usually conducted every spring and summer.

The summer exercise, Ulchi Freedom Guardian (UFG), consists mainly of computer simulations — however the spring exercises, code-named Foal Eagle and Key Resolve, are a fair bit more action-packed.

Military exercises involve live-fire drills with tanks, aircraft and warships. ( AP: Lee Jin-man, file )

They usually involve live-fire drills with tanks, aircraft and warships.

The Australian Defence Force also took part in last year's summer exercises, which North Korea described as a "suicidal act" on the part of Australia.

This year's drills were postponed until April so as not to overlap with the Pyeongchang Winter Olympics Games and to help reduce tensions for a resumption of talks between North and South Korea.

A Pentagon spokesman said in March the two joint drills — Foal Eagle and Key Resolve — would involve about 23,700 US troops and 300,000 South Korean forces.

What is their real purpose?

Sorry, this video has expired Flurry of military drills on Korean peninsula in show of force against North

While it's important that allied militaries are able to spring to action in a coordinated fashion at short notice, war games also serve as a strong deterrent to potential adversaries.

Sending scores of tanks, warships and aircraft to the Korean Peninsula — and showing that you know how to use them — is just about the strongest warning the US and South Korea could give North Korea.

"You deter a potential adversary by showing that you've got forces that are ready, willing, and capable to act," Mr Shoebridge said.

"And that's probably the biggest purpose, because the goal is to deter conflict rather than engage in it."

Bombs dropped by US and South Korean jets hit a simulated target during a joint exercise. ( AP: South Korea Defence Ministry )

In terms of the Korean Peninsula, the high-profile drills have also been used as a statement of unity against the North.

Last September, the United States military staged bombing drills along with South Korea over the peninsula after Pyongyang fired a mid-range ballistic missile over Japan and conducted its sixth and most powerful nuclear test.

Why does North Korea hate them?

North Korea describes the exercises as "provocative", and now Mr Trump agrees. ( AP: Evan Vucci )

The United States, South Korea and their allies have previously always described their joint exercises as purely defensive measures.

However, North Korea has long considered them to be preparations for an invasion.

The country has in the past conducted weapon tests of its own during the drills, for example the missile test that flew over Japan in August last year coincided with the summer US-South Korea exercises.

Sorry, this video has expired Trump says war games with South Korea have been suspended

After the Max Thunder military exercises held this May, North Korea suspended high-level talks with South Korea and threatened to walk out on the planned summit with Mr Trump.

North Korean state media described the Max Thunder air combat drills as "provocative" — a line of rhetoric Mr Trump echoed this week when announcing the end of joint-military exercises in the region.

Mr Shoebridge said North Korea's desire for the joint exercises to end was proof the drills had been serving their purpose.

"The fact that [Mr Kim] could see that they were working effectively with the South Korean military, and were able to respond at short notice should he use his forces, worried him," he said.

What are the consequences of stopping them?

Mr Trump's decision to stop the war games is generally seen as a significant concession to Mr Kim, without the US getting anything solid in return.

"They've handed the initiative to Kim Jong-un, and also they've taken away that really important deterrent to Kim and left him with a free hand to act if he wants to," Mr Shoebridge said.

"So reversing that suspension is an active thing that now Trump has to do, when before, he held the cards."

Ending the US-South Korea war games in exchange for the North to stop its nuclear and missile tests is also what China has always wanted.

And the proposal — "suspension for a suspension" — is exactly what Mr Trump conceded.

Experts say this is a signal to China that their demands are being met, and could be a golden opportunity for the middle kingdom to consolidate its ambitions of dominance in Asia.