From Admiral James Stavridis, USN, Ret.

I commanded dozens of major bases in Europe for 4 years as Supreme Allied Commander of NATO & Commander, US European Command. We had plenty of scares, but nothing that required this level of alert. Clearly there is a deep concern and that is consistent with everything I can see.

https://www.defense.gov/News/News-Stories/Article/Article/3823705/us-bases-in-europe-go-to-higher-state-of-alert/