President Barack Obama has indicated this week he will consult international law before responding to the gas attacks in Syria, but experts say any U.S. military strike against the regime of Bashir al-Assad would violate the very same laws.

These laws of war go back to the early 20th century and govern who gets to start a war and how they are to be waged. But they are often stretched by leaders who need them to pursue their own political goals or perceived security interests.

In 1999, President Bill Clinton cited humanitarian reasons and in particular, a massacre of 45 people in Kosovo, for a NATO-led bombing campaign of Serbia that did not have the support of the United Nations. The perpetrators of the massacre were believed to be Serbian, but there has been dispute since then about what really happened in the village.

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In 2003, President George W. Bush claimed Saddam Hussein possessed weapons of mass destruction -- including chemical weapons -- as justification for attacking Iraq. He sent then-Secretary of State Colin Powell to the United Nations to convince the body to act. The U.N. didn’t, and the United States invaded anyway. Later, the weapons claims were disproven by the president’s own Central Intelligence Agency.

The United Nations Charter forbids countries to attack each other unless in self-defense or if authorized by a vote of the U.N. Security Council. That happened in 2011, when the council agreed to allow air strikes against Libyan government forces to protect a potential massacre of civilians in Benghazi.

The United States claimed self-defense in invading Afghanistan to hunt for the Al-Qaeda operatives responsible for the 9/11 attacks on Washington and New York.

President Obama is a constitutional lawyer, and likely will be looking at these past cases. But experts say he doesn’t have much of a legal leg to stand on.

“As of now there is no legal justification for an attack,” said Michael Shank, director of foreign policy at the Friends Committee on National Legislation and professor of conflict analysis and resolution at George Mason University. “The only way that you can use international law to justify an attack on Syria is if the U.N. Security Council votes in favor.”

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What about the use of chemical weapons? The White House has said that it believes government forces in Syria were responsible for the latest chemical attack that killed 355 people in the outskirts of Damascus. A team of U.N. weapons inspectors are on the ground this week trying to find out what happened and who did it. Their report likely won’t be completed until at least next week.

Still, it turns out that Syria never signed or ratified the international chemical weapons convention of 1992 that bans their production, stockpiling or use. (Neither did Egypt or Iran; Israel has signed but not ratified it).

The use of chemical weapons is also banned by the Geneva Conventions, a series of laws governing the conduct of warfare that were first drafted in 1864 and then updated in 1949. The conventions prohibit the killing of civilians, wounded troops and hospitals, or attacking opposing forces while flying a white flag of surrender. They also prohibit the use of chemical gas or weapons, such as a flamethrower, that inflict horrendous pain on their victims.

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Even if they are often flouted, the Geneva Conventions govern the conduct of troops in battle -- but not the reason for going to war in the first place, according to Mary O’Connell, professor of international dispute resolution at University of Notre Dame.

“The president seems to be putting the United States in the position of violating a very important law because he is so angry about Mr. Assad’s violation of the same international law,” O’Connell said.

O’Connell said that in the 1980s, Saddam Hussein used gas on both Kurdish civilians living in Iraq, as well as Iranian troops attacking his country. He also gassed his own troops, invited in U.N. inspectors, and tried to convince them that Iran used the chemicals. It didn’t work.

O’Connell believes the current U.N. weapons inspectors’ report may clear up some of the questions. Late Thursday, French officials said they wanted to wait until the report is complete before agreeing to join any U.S. military action.

“I hope they take all the time they need because so much is riding on this,” O’Connell said. “They have to be very, very cautious.”