WASHINGTON (AP) — President Barack Obama is poised to become the first U.S. leader in three decades to attack a foreign nation without mustering broad international support or acting in direct defense of Americans.Not since 1983, when President Ronald Reagan ordered an invasion of the Caribbean island of Grenada, has the U.S. been so alone in pursuing major lethal military action beyond a few attacks responding to strikes or threats against its citizens.It's a policy turnabout for Obama, a Democrat who took office promising to limit U.S. military intervention and, as a candidate, said the president "does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation."But over the last year Obama has warned Syrian President Bashar Assad that his government's use of chemical weapons in its two-year civil war would be a "red line" that would provoke a strong U.S. response.So far, only France has indicated it would join a U.S. strike on Syria.Without widespread backing from allies, "the nature of the threat to the American national security has to be very, very clear," said retired Army Brig. Gen. Charles Brower, an international studies professor at Virginia Military Institute in Lexington, Va."It's the urgency of that threat that would justify the exploitation of that power as commander in chief — you have to make a very, very strong case for the clear and gathering danger argument to be able to go so aggressively," Brower said Friday.Obama is expected to launch what officials have described as a limited strike — probably with Tomahawk cruise missiles — against Assad's forces.Two days after the suspected chemicals weapons attack in Damascus suburbs, Obama told CNN, "If the U.S. goes in and attacks another country without a U.N. mandate and without clear evidence that can be presented, then there are questions in terms of whether international law supports it; do we have the coalition to make it work?" He said: "Those are considerations that we have to take into account."Lawmakers briefed on the plans have indicated an attack is all but certain.