Lol, no it isn't. Ever since Cold War times, the law has been that the president can not only launch missiles, but even deploy troops on the ground, without congressional approval. There's a limit on how long the troops can be deployed, iirc, but once you've started a ground invasion it's a bit late.

Obviously it's unconstitutional, but there's no legal precedent that would say so. If you think this is the first time this has happened, you really need to learn more about history. The US never formally declared war on Vietnam, for example.