“What history demonstrates is that an act of war exists when a country says it does,” according to Gary Solis, a retired U.S. Marine who teaches at West Point and Georgetown University Law School. “There’s no international body to referee and say, ‘This is war.’ So it becomes political.”

For example, Solis says, “nobody ever said anything about war” when the U.S. Army sent 4,800 soldiers into Mexico in 1916, commanded by Maj. Gen. John J. Pershing, to capture Francisco “Pancho” Villa in retaliation for an attack on a town in New Mexico. …

Says Prof. Robert Goldman of American University’s Washington College of Law, “You have border incidents occurring all the time, such as between India and Pakistan. Neither side, however, has instituted hostilities.”

In Ukraine, he said, shots could be fired between Russian and Ukrainian soldiers. “But it’s going to be a political decision whether the two sides are going to want to treat this as an act of war.”