The powder keg that is the Middle East will not begin another World War, at least so long as individuals from the neighboring countries and countries further away remain rational about their interference in the Syrian Civil War. At present, we have no idea just who ordered the launch of weapons in Damascus, and I don't think the chemical analysis done by the UN's agents has come in/been announced, yet.
If the US engages in a war without knowledge of who used the chemical weapons and without a UN sanction, then it risks playing the roll of aggressor. If a UN sanction is issued to begin strikes against Syria, then it will be over fairly quickly. The only foreseeable route that would lead to a massive war is if A) somebody engages without support from the UN, B) some do not get involved despite a request from the UN or C) the UN becomes entirely disregarded.
To be completely honest, the US isn't really healthy enough to keep joining wars. Let's sit this one out, unless damning evidence arises that the present regime in Syria engaged in chemical warfare with its own people, which we won't know for a while yet.
