The United States and South Korea have signed a joint plan to counter what they call North Korean provocations.
The new agreement sets out a contingency plan for US and South Korean forces, in the event the North launches an incursion or a limited attack. It builds on the two militaries' existing joint command.
Help from the US - which has 28,000 troops in South Korea - has until now been optional in minor skirmishes.
Regional tension remains high after the North's third nuclear test last month.
The BBC reports the US already offers South Korea a "nuclear umbrella" but Cold War experts have pointed out that while nuclear deterrence may address the possibility of all-out war, it does not deter low-level incidents.
Under the new plan, South Korea will be able to call on US assistance should Pyongyang follow through with its recent threats, for example to attack remote South Korean islands.
"This allows both nations to jointly respond to the North's local provocations, with the South taking the lead and the US in support," South Korean defence ministry spokesperson Kim Min-seok said on Monday.
"It will have the effect of preventing the North from daring to provoke us," he said.
The deal was signed on Friday.
