The U.S. has relied on NATO at least three times to give it broad foreign support for military missions: in bombarding Bosnia in 1994 and 1995, attacking Kosovo with airstrikes in 1999 and invading Afghanistan in 2001.
Only a few times has the U.S. acted unilaterally — and only then to respond to attacks or direct threats against Americans.
In 1986, Reagan joined ordered airstrikes on Libya to punish then-leader Moammar Gadhafi for the bombing of a Berlin dance club that killed two U.S soldiers and wounded 79 other Americans.
Three years later, George H.W. Bush invaded Panama after dictator Manuel Noriega declared war on the U.S. when his drug-trafficking regime was slapped with crippling American sanctions. The invasion began four days after a U.S. Marine was killed in a shooting in Panama City.
Clinton ordered a missile strike against Iraq in 1993 as payback for an assassination attempt against the elder Bush. And in 1998, Clinton attacked al-Qaida bases in Sudan and Afghanistan to retaliate against U.S. Embassy bombings in Kenya and Tanzania that killed more than 200 people.
Obama approved the 2011 raid that killed Osama bin Laden, who had been considered a threat potentially going back to the 1996 bombing of the Khobar Towers in Saudi Arabia that killed 19 U.S. troops living there. Additionally, the U.S. has launched hundreds of deadly drone strikes on suspected al-Qaida havens, mostly in Pakistan, Afghanistan and Yemen during the presidencies of Obama and George W. Bush.
All other major U.S. military attacks since the 1983 Grenada invasion have been sanctioned by the United Nations. That includes the 2011 missile strikes that Obama ordered against Libya as part of a coalition to protect that nation's citizens by enforcing a no-fly zone against Gadhafi forces.
Even the Grenada invasion had some international support. Six Caribbean island countries asked for U.S. intervention, which the Reagan administration said was legal under the charter of the Organization of American States. But the invasion was roundly criticized by Britain, Canada and the U.N.