Why did the farmers grant congress sole power to declare war? What tensions may arise from the division of war powers?

Declaring war is something that puts the lives of the American people at risk. As such, they choose to entrust that decision to Congress instead of just one person. Since we do have a standing army, the President can effectively wage war without Congress going along with it (The Vietnam War). Since he controls the army and can just order them to go into combat mode.

The division of war powers between Congress and the President has never been free of tension. The constitution grants Congress the power to declare war, to raise and support armies, and to make rules for the regulation of those armed forces. The President, on the other hand, is the Commander and Chief of the U. S. armed forces.