The plan lays out how the allies will communicate with each other and react to any future aggression from North Korea.
It provides for a joint response from the two countries, even if there is a limited attack from the North.
North and South Korea are still technically at war after signing an armistice, not peace treaty. Previously, help from the US and its 28,000 troops in South Korea was optional in minor skirmishes.
In the event of a large-scale resumption of war, according to GlobalPost senior correspondent in Korea, Geoffrey Cain, the US will assume operational command over all forces in South Korea.
"That means the 8th Army, which is based in Seoul, will have the ultimate say over both the movements of American and South Korean troops," Cain said from Seoul, adding:
Today, the US and South Korea signed a different type of plan that could enlist Washington in small-scale attacks from North Korea.
The contingency plan is called "South Korean-led, US-supported," and has been put on paper in order to serve as a deterrent against North Korean provocations. The allies refused to divulge further details on how the US would function in its supporting role.
Pyongyang is basically waging a guerrilla war, according to GlobalPost's Cain, using rickety old equipment against the technologically superior South Koreans.
"So it's understandable why the North has occasionally launched quick, containable naval or artillery strikes in, say, the West Sea, known to Americans at the Yellow Sea — because they know they'd lose a large-scale war," Cain said.
Washington has always held back Seoul from responding to the North's provocations with military force. Now that the US can get involved, the balance of power could be shaken up at least a little.
And that's exactly what South Korea has been trying to contain. BBC notes:
The "provocative" acts that the plan seeks to address include incursions on the border and by low-flying aircraft, and attacks on border islands, says the Agence-France Presse news agency.
Tensions between the two Koreas are now at a high since Kim Jong Un's regime detonated a nuclear device in February. The US and South Korea have been conducting military drills that North Korea considers a threat.
The northern nation has said the actions of the allies have put the peninsula on the brink of war, with the Obama administration boosting its anti-missile defenses as a result.