It is unquestioned national support and national mobilization something that hasn't happened since WWII. In that war everyone eligible for service got called up. Private industry was re-geared for war production. Women went to work in factories. Civilians were rationed food and gasoline. And it was all paid for with increased taxes, sale of War Bonds and rationing. Everybody was in the war for the duration. It got done and done quickly. Since then, only the military gets involved in the wars, the civilians suffer nothing and the politicians get have their cake and eat it too. The result is wars that don't have the support of the people, foolish wars that cost a fortune in borrowed money and lasted for years and years, bleeding us out. A declared war involves the entire country and is not taken lightly. Now, this doesn't prevent the Executive from using military power in the short term to protect the interests of the country. But beyond a certain level of commitment, with consideration of time and expenses, the Congress should get involved so that there is no suggestion that it is the war of a certain individual or a specific political party, but a war of national best interests. They are not. Wars range from colossal to tiny and in every flavor. One size does not fit all. Some wars require decades of planning, while others are "come as you are". Some wars are very smart and other incredibly stupid. Some are astonishing successes and others are dismal failures. Some are just, some are unjust. This is why we have established a balance point to enable the President to deal with immediate brushfires, yet allows Congress to consider all factors in the best national interests before committing to a major war effort.
A covert assassination does not require a war. It's a simple military action requiring little time, few expenditures, low risk, and no ongoing expenses. It's why we have a Supreme Commander.