PARIS—President François Hollande's vow to join a potential U.S.-led strike on Syria propels France to the unusual position of Washington's top ally after the U.K. withdrew amid strong public and parliamentary opposition to military action.

If the U.S. and France carry out strikes on the regime of Syrian President Bashar al-Assad, the tandem would mark a sharp turnabout a decade after the two countries clashed over Washington's decision to invade Iraq and topple the regime of Saddam Hussein.

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