Which branch can declar war?
In the United States, the legislative branch is the only branch of government that can declare war. The legislative branch is represented by the Congress.
According to the U.S. Constitution, the power to to declare war is vested solely in Congress. See Article I Section 8 The Congress shall have power "to declare war, grant letters of marque and reprisal, and make rules concerning captures on land and water." The president can ask Congress to declare war, but cannot do so himself.
"In the United States, the power to declare war and appropriate military funds lies with the legislative branch of government, but the executive branch supplies the commander in chief of the military, capable of some military actions even without an official declaration of war." Quote from Wikipedia.
The combination of three main events, that were; Unrestricted Submarine warfare from Germany's behalf, the zimmerman telegraph, which was an incident where Germany sent a telegraph to Mexico asking them to deaclare war upon the U.S. and the sinking of the Lusitania in 1915 killing 128 U.S. and an additional 4 U.S. passanger ships carring supplies to Britain for the war effort. For these reasons the U.S. based their grounds to start war, and declar…