The U.S. Embassy in Qatar issued a warning for American citizens to shelter in place due to security concerns. This caution came after the U.S. joined Israel in attacking Iran's nuclear sites. Iran had threatened retaliation if the U.S. intervened in the conflict. The U.S. has a significant military presence in the Middle East, including the largest base in the region located in Qatar. President Trump recently visited this base, which houses around 8,000 U.S. troops.
Source: https://www.cbsnews.com/news/us-embassy-qatar-americans-shelter-in-place-iran-israel-war/