Final answer:
Congress is the entity of the US government that has the power to declare war.
Step-by-step explanation:
The entity of the US government that has the power to declare war is Congress. According to the US Constitution, specifically Section 8 Clause 11, only Congress can officially declare or make war. The President does not have the power to declare war, although they can serve as the commander-in-chief of the armed forces. It is worth noting that Congress also has the power to limit the President's authority in terms of declaring war or providing funds for the military.