The answer might seem obvious: one, at least since the withdrawal of U.S. troops from Iraq. But Harvard's Linda Bilmes and UCLA's Michael Intriligator argue that it's at least four:
In addition to these two large-scale conflicts the US is also fighting a number of unannounced and undeclared “wars”. These unannounced wars are fought mainly with air power and increasingly with drones rather than ground troops. If we define war to include conflicts where the US is launching extensive military incursions, including drone attacks, but that are not officially “declared,” then the US is directly involved in at least three wars – in Pakistan, Yemen, and Somalia – in addition to Iraq and Afghanistan. These unannounced wars follow in the tradition of many previous covert US military incursions, such as in Chile, Cuba, and Nicaragua. The difference is that advanced military technology now enables the US to fight such wars in a different way, which is far less transparent, and to sustain operations over several years.
As James Fearon notes at Monkey Cage, the article raises the question of whether a "war" requires that both sides experience casualties, or merely that both sides be involved in military activity of some sort.
It is true, though, that the relatively small portion of the U.S. population needed to participate in military action has blurred the line between peacetime and wartime. Even the official war the U.S. is fighting -- Afghanistan -- is hardly topic A for the U.S. government or media these days. Afghansitan isn't even the nation's most discussed foreign policy issue, something unthinkable in the era where a draft or at least mass mobilization were needed to fight wars.
When U.S. troops withdraw from Afghanistan, it probably won't feel like a major transition for most Americans. That certainly doesn't mean we should feel nostalgic for the days when wars involved nationwide mobilizations and massive casualties, but the new state of affairs certainly doesn't put much pressure on political leaders to end miltiary conflicts -- whatever you call them -- once they've begun.
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