Interestingly, it isn't likely to have much effect at all, according to most pundits, and if it does have an effect, it will likely benefit your portfolio of domestic securities. And the professionals at Neikirk, Mahoney & Smith CPAs want you to get the best advice possible.
According to Mark Arbruster in Inside Investing, "With the stock market at an all-time high, we are frequently asked about the situation in Syria, and whether now might be a good time to beat a hasty retreat from stocks. To address these concerns, we reviewed historical capital markets performance during times of war."
Certainly worth a read at http://blogs.cfainstitute.org/insideinvesting/2013/09/25/u-s-capital-market-returns-during-periods-of-war/