Secretary of State John Kerry told senators last week that a "right of hot pursuit" could provide a basis for military forces to move across the border between Iraq and Syria to strike at Islamic State militants. But does Kerry's legal theory — which has little grounding in international law — provide firm precedent for Monday night's massive U.S. airstrikes in Syria and for future military actions?

President Barack Obama has said repeatedly in recent days that U.S. troops will advise Iraqi forces but will not be used for combat directly against the Islamic State group. During a hearing last week by the Senate Foreign Relations Committee, Kerry said the same, but then unexpectedly explained the hot pursuit doctrine, which had not previously been cited by anyone in the Obama administration to legally justify any part of a new war.

"So, Iraq is asking us to help them," Kerry said. "And as a matter of right, if they're being attacked from outside their country, you have a right of hot pursuit. You have a right to be able to attack those people who are attacking you as a matter of self-defense."

International law experts said there is a recognized right of hot pursuit to pursue ships escaping in international waters, but there is no clear global legal authority that would allow one nation to violate another nation's border to apprehend an opposing force on land. Even without that precedent, numerous nations have repeatedly taken action across borders — including raids by U.S. troops in recent years pursuing militants from Afghanistan into Pakistan.

A State Department spokesman, Jeff Rathke, elaborating on Kerry's comments last week, said Kerry "was referring to self-defense, which includes the right to use necessary and proportionate force to address armed attacks that emanate from another nation, if that nation is unwilling or unable to address the threat."

Neither Kerry nor his spokesman identified which military forces, Iraqi or American, might rely on such a legal basis. But Kerry's comments, which came during an exchange last Wednesday with Sen. Ben Cardin, D-Md., closely followed a discussion about the role U.S. and Iraqi forces and an international coalition would play in countering the Islamic State threat. Cardin had asked Kerry how the government should obtain congressional approval for a war that would also "protect us against any lengthy particularly combat involvements in these countries in the future."

U.S. forces used fighter jets, bombers and Tomahawk missile strikes against Islamic State targets in northern Syria on Monday, Pentagon officials said. Officials also said Bahrain, Qatar, Saudi Arabia, Jordan and the United Arab Emirates took part in the airstrikes.

Even recognizing that nations have repeatedly invoked their self-interest in striking at opposing forces across borders, legal experts said there is no governing legal code that sanctions a right of hot pursuit on land.

Temple University law professor and international law authority Peter J. Spiro said the hot-pursuit doctrine is well-established in criminal law, used to justify U.S. law enforcement pursuit of an armed fugitive across a border or state lines. But that doctrine is "contested when it comes to the use of military force," he said. Spiro added that "without some justification or U.N. National Security Council authorization, any use of force will comprise a violation of Syrian sovereignty."

There is clearer authority when it comes to pursuit on the sea. The 1958 Geneva Convention codified authorities' right to pursue and apprehend ships that have violated a nation's laws and have escaped from a country's national waters into international waters. Kerry cited that law in 2008 when, as the incoming chairman of the Senate Foreign Relations Committee, he called for the pursuit of pirates onto land in Somalia. The United Nations Security Council later authorized sea-to-land pursuit in Somalia.

Even without any clear legal grounding for hot pursuit on land, the U.S. has previously done it when officials believe it's in the national interest. President Woodrow Wilson ordered an expeditionary force of 4,800 troops under Maj. Gen. John J. Pershing to pursue Mexican revolutionary Pancho Villa in 1916 into Mexican territory. U.S. soldiers were sent on raids into Cambodia and Laos during the Vietnam War.

And U.S. forces have struck at Taliban militants in Pakistan from positions inside Afghanistan. In March 2007, U.S. Army Lt. Gen. Douglas Lute acknowledged that American troops had crossed from Afghanistan into Pakistan to attack retreating Taliban forces.

"If they demonstrate hostile intent — so, for example, if just across the border inside Pakistan we have surveillance systems that detect a Taliban party setting up a rocket system which is obviously pointed west into Afghanistan — we do not have to wait for the rockets to be fired," Lute said at a congressional hearing. "They have demonstrated hostile intent and we can engage them, and by the way, we have."

As a Swift Boat commander during the Vietnam War, Kerry practiced a version of hot pursuit on his own, beaching his boat to pursue Vietcong guerrillas firing from land. In one incident, Kerry shot and killed a Vietcong armed with a weapon and was later awarded the Silver Star — even though his superiors had ordered boat commanders not to risk combat on land.

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Associated Press writer Matthew Lee contributed to this report.