Should the United States be ready to fight two wars at the same time?
A lot depends on how one defines “fight”, “two wars”, and “at the same time”.
During its early history, the United States, according to many definitions of these terms, did not fight as many as two wars at the same time. During the 20th century, according to other definitions, the United States did not fight as few as two wars at the same time. I doubt that it was ever historically necessary for the United States to be able to fight exactly as many or as few as two wars at the same time. Based on past performance, this metric does not appear to be particularly useful as a guide to present or future policy over changing conditions.
A more useful maxim, derived from classical international relations theory, is that the United States should have the capabilities to defend its vital national interests. One of its vital national interests, as President Eisenhower emphasized, is to maintain a healthy economy. While national security is important, so also is the security of national citizens.
American military capabilities should change, as American interests and the nature of international threats and violence also change. It always has been and always will be necessary for the United States to be able to fight as many or as few wars as may be crucial to American vital national interests.
There is no magic number.
Francis A. Beer 