The conflict between Russia and Ukraine has hit western European stock markets especially hard. While the US suffered just a day or two of declines following the initial escalation of the conflict at the end of February, Europe has been on a straight path lower. You can see the divergence between the US and Europe in the six-month charts of the S&P 500 and the German DAX. The circled area represents the same time frame over the past few weeks.
Both the US and Germany experienced decent pullbacks in late January into early February, and they both bounced sharply off of the lows in mid-February. But while the S&P 500 has managed to hold onto a large bulk of its post-low gains, the DAX has already taken out its prior lows.
Up until Thursday, US investors had mostly shrugged off Europe's pullback, but Thursday's technical breakdown in Germany seems to have awoken the bears.
Given how global US companies are these days, the major indices are going to get hurt on weakness in Europe. One thing you can do in these instances, however, is stay away from companies that generate a large portion of their revenues outside of the US and focus on buying more domestically-focused names.