Lance is lying to you
Congress holds the power to declare war. As a result, the President cannot declare war without their approval. However, as the Commander in Chief of the armed forces, Presidents have sent troops to battle without an official war declaration (which happened in Vietnam and Korea). The 1973 War Powers Act attempted to define when and how the President could send troops to battle by adding strict time frames for reporting to Congress after sending troops to war, in addition to other measures, however it has not had much effect (see "War Powers Resolution" section in the Commander in Chief Powers article).