Inherent powers are powers that a particular branch of government holds because they are necessary to perform its duties. The three branches of government under the US Constitution are the Legislative Branch (Congress), the Executive Branch (headed by the President), and the Judicial Branch (the Supreme Court and the lower courts).
A US naval vessel is attacked in the Pacific Ocean. Without a declaration of war from Congress, the President orders a retaliatory strike. The President argues that they can order this action without a declaration of war because, as Commander-in-Chief of the Armed Forces, the President has the inherent power to make individual military decisions without congressional approval. On the other hand, if the President were to declare war, that would fall outside their inherent powers because the Constitution reserves to Congress the authority to declare war.
For more detailed information, see our related Constitutional Law terms: