Answer:
Congress needs to be notified of troop deployments and must approve long-term military actions.
Passed in 1973 over the veto of President Richard Nixon, the War Powers Resolution (its official name) blocks presidents from continuing the pursuance of a war without Congress's approval. Congress must be notified within 48 hours if troops are deployed, and troops cannot remained deployed beyond 60 days without approval from Congress.
The US Constitution leaves some tension in place between the legislative and executive branch when it comes to the country's involvement in war. Article I of the Constitution gives Congress the power to declare war. But Article II names the President the Commander-in-Chief, and presidents frequently have understood that role as containing the authority to deploy US forces without first getting congressional approval. The War Powers Act requires the President to notify Congress of troop deployments and limits the length of time troops can be deployed without Congressional approval.
