The Obama administration is lobbying hard for a congressional resolution authorizing the use of force. Success is not guaranteed, but even if Congress does approve intervention the fundamental dynamic at the United Nations and broader international stage would remain unchanged.
The USA would still be going it alone — America’s major allies, regional partners like the Arab League, and the Security Council are not backing the action. It is not as if a vote in congress will somehow sway Moscow at the Security Council; and I doubt a congressional vote will change the minds of British parliamentarians who rejected a strike just last week.
The authorization of the use of military force may be the legal (and proper) thing to do here in the USA. But congress’ vote has no bearing on whether or not this intervention meets the standards of international law. Only the Security Council can do that. The vote also will not change the underlying problem that a US-lead intervention will be basically unilateral and lack broad international support. Congress’ vote will not sway the Arab league or even a decent cohort of America’s international allies to join a coalition of the willing. The USA would still be essentially unilaterally embarking on this mission, which undermines the principles of collective action and collective security that underpins the modern international system.
The day after congress votes, a US-lead intervention would still be technically illegal; it would also not garner broad international support that might confer at least a fig leaf of legitimacy to unilateral american military strikes.