When the people of a country arise in a popular revolution against oppressive dictators who use their powerful military to destroy the rebels, then the international community should study the problem and act in a measured and united way to rectify the situation. Actions may include trade embargoes, medical aid, food aid, military materiel aid, no fly zones, ground, naval or air support, or any combination of these under UN Command.

Sadly, the UN has failed to be the medium for this because of the veto power given to the permanent members of the Security Council. Since the power to use UN military force is vested in the Security council and two permanent members of the SC favour dictatorial forms of government, then the UN can never work in the ways that its founders intended it to do.

It is not necessary to scrap the UN, and that would be bad because its various specialized organisations do a great deal of good. However, it has become necessary to scrap the Security Council and start again with something better. There should be no permanent members and no veto powers. A majority vote of the new SC should be sufficient for military intervention in civil wars.

The current permanent membership of the Security Council reflects the power of the victorious Allies existing at the end of WWII, plus France (which played on both sides during WW II) for reasons that seem to be more to do with its pride, culture, and history. Stalin and Churchill had undue influence at that time. A great deal has happened since then, so it is highly appropriate that the SC should be reviewed and revised to reflect the current world realities.

My proposal!
Membership of the new SC should be by the 8 regions of the world: North America; Europe; South and Central Asia; East Asia; Africa; South America; Arab League; and Oceania. These Regions should decide which country represents them for limited terms and should debate civil war intervention proposals among themselves. No Region should have a veto power.

Each country member should be free to decline to support any military intervention through the use of its own forces, and any country could opt in, even if its Region voted against. I think that the USA would continue to have a place on the new SC because the world would want it to commit resources to any interventions.

So in my world I think the Syrian case would play out that the USA, Russia and China might convince their regions to vote against (I doubt this would happen but they could try). The vote then would be 5-3 in favour, and support to the rebels would be provided by Europe, Arab Countires and Australia and NewZealand. Canada would probably opt in.
I think that would be enough. In this way the USA could make up its own mind on each case, and be as selfish as it considered necessary in the particular circumstances. Sure, the USA would have less influence in the SC but what it has now is not much use if two other countries can veto it.