There are different thoughts on this, but most investors like to have some international exposure (in addition to the exposure you get from American businesses that do business overseas). Most recommendations are for 20% to 40% of your stock portfolio be international.
That being said, John Bogle has said international funds are not vital to a portfolio, and Buffet has said something similar.
Current valuations make international look more attractive right now, with significantly lower P/E ratios than American stocks.
Whatever you do, pick an asset allocation and stick to it. If international underperforms for the next year or 2, rebalance into it, don't abandon it. Take advantage of buying low and selling high with proper rebalancing.
Best,
-PoF