The authors of the Constitution went through considerable effort to make sure the war-making power was vested in the legislative branch, rather than the executive branch, and for the first century and a half of the Republic that formulation held.
Unfortunately, in 1950 we started an experiment in the alternative when Harry Truman committed U.S. troops to a major war in Korea (styled a "police action" at the time) without a declaration of war by Congress. As a matter of fact, Congress has not declared a state of war since, although there's been no shortage of fighting in that time.
Now, human affairs are not subject to "proof" in the scientific sense as no controlled experiments being possible. But it's certainly reasonable to examine the evidence we have to determine whether certain polices are successful or not, and a period of nearly six decades seems more than adequate. It only took about that long to demonstrate, for example, that communism was a failed system.
In the first 160 years of the Republic we fought five foreign wars, with one draw and four clear wins. (War of 1812, Mexican-American, Spanish American, World War I and WWII.) This averages one war every 32 years and an 80 percent success rate. In other words, wars were infrequent and successful. This seems a generally desirable state of affairs I think everyone will agree. During that time the United States grew in power and influence from a relatively weak minor power into the most powerful single country on earth. This period I'll call the Congressional War Era
(I don't count the Revolution because it was prior to the adoption of the Constitution and also because it was a domestic war, which is an altogether different animal than a foreign war. For the same reason I leave off the Civil War, another domestic conflict that has its own sets of legal, moral and practical issues wholly different than the problems of foreign wars).
In the most recent 57 years of the Republic we've fought four major foreign wars (Korea, Vietnam, Gulf and Iraq/Afghanistan). Of those, one was a draw, one was a defeat and one a clear victory. The last is ongoing, but prospects for a clear victory are about nil. This averages a war every 14 years and a success rate of 25 percent. In other words, wars have been more than twice as frequent while being much less than half as successful. I think everyone can agree this is not a desirable state. I'll label this the Presidential War Era.
During both periods the executive branch engaged in minor military actions with mixed success. Because these were, by definition, minor actions however, the risks and payoffs were relatively low. Failures were expected and tolerated. While there were individual tragedies, of course, the country as a whole was largely unaffected by adventures in Cuba, Hawaii, the Philippines, Haiti, Panama, the Barbary coast, Libya, Lebanon, Grenada, etc.)
On the other hand, major foreign wars unavoidably bring great risks and the certain expenditure of national treasure in the form of lives and money.
The evidence is clear that the Congressional War system provides superior results to the Presidential War system. Why this is so will be addressed in a future post, but the empirical evidence is there for all to see.