Image copyright AP Image caption Senator John McCain (left) has called for US intervention in Syria for months

In a heated phone call between Secretary of State John Kerry and Democratic members of Congress, one of them warned that the administration had "collective amnesia" about the lessons of Vietnam.

In Vietnam what started with the US backing one side and sending in a few military advisers ended up in a long bloody war.

This is not the same. Lessons have been learnt. Every American politician from Mr Obama down is very clear that there will be "no boots on the ground" in Syria.

But it already seems Mr Obama is changing his strategy, deepening his commitment, because of his decision to call a vote in Congress.

Republican Senator John McCain emerged from the White House, if not convinced, then on the way to being satisfied.

His argument has always been that "pinprick" strikes were pointless symbolism - Mr Assad had to be weakened.

He suggested Mr Obama had given hints that the plan was more serious, that it would degrade Mr Assad's ability to fight, and beef up the rebels. Not Mr Obama's shot across the bows, but sustained fire into the ship itself, leaving it crippled.

Today's hearing in the Senate will be critical - a chance for Mr Kerry, Defence Secretary Chuck Hagel and Chairman of the Joint Chiefs of Staff General Martin Dempsey to put more flesh on the bones, and explain what they want to achieve in Syria.

But it would be ironic if Mr Obama's decision to get congressional backing forced him further down the road to war than he thought was wise last week.