Does Russia's military adventure in Ukraine amount to an act of war? And if not, what does the term mean?
Several Western nations, including the United States, Canada and the United Kingdom, have described Russia's actions in Crimea as an act of aggression.
Canadian Prime Minister Stephen Harper said it was "a clear violation of Ukraine's sovereignty and of international law." Ukraine's Prime Minister Arseniy Yatsenyuk went further, calling it "an act of war."
Surprisingly, legal experts say what constitutes an act of war is in the eye of the beholder.
"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.
By contrast, Japan declared war on the United States in 1941 and bombed the Navy base at Pearl Harbor, Hawaii, after the US imposed an oil embargo, not in retaliation for a military action.
In its declaration of war, Japan cited "the direct severance of economic relations, menacing gravely the existence of Our Empire."
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."
Formal declarations of war, for all but rhetorical purposes, ended with the adoption of the United Nations Charter in 1945.
"All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any State," the charter said.
But it took 29 more years for the U.N. to formally define what's now viewed as the international standard — an act of aggression.
Among the actions listed: "the invasion or attack by the armed forces of a State of the territory of another State, or any military occupation, however temporary, resulting from such invasion or attack."
In practice, the definition has rarely guided the U.N. Security Council in determining when aggression has been committed.
"It has now taken on a new life as a source for discussion of the definition of the individual crime of aggression within the jurisdiction of the International Criminal Court," writes Professor Elizabeth Wilmhurst of University College, London, in a U.N. legal treatise.
Since the U.N. Charter was adopted, "the only time armed conflict is lawful under international law is in self-defense or when the Security Council authorizes it," Goldman says.
Some legal experts say even though no shots have been fired, Ukraine has a potential case that Russia has already committed an act of aggression, given its show of military force and takeover of part of another nation’s territory.
“Most importantly, consider the implications for the charter if the answer were that this were not an armed attack: Ukraine could not lawfully use force against Russian troops to protect territory that undisputedly is part of Ukraine,” writes Ashley Deeks of the University of Virginia Law School on the legal blog “Lawfare.”
Other legal scholars, however, say disputes about the legitimacy of Ukraine’s interim government may weaken its case.
The use of military force is central to the U.N.'s definition of aggression. It says nothing about the employment of economic sanctions, the course the United States has so far pursued, in addition to diplomatic efforts.