The United States moved Belgium to Level 2 of its four step travel advisory scale on 23 July, asking Americans to exercise increased caution. The reasons given were petty crime in tourist areas, terrorist violence, and civil unrest. Level 4, at the top of that scale, is the instruction not to travel at all.
Those three sit on one line and they are not the same kind of thing. Pickpocketing and bag snatching around a busy station or a crowded square are close to a certainty in any European capital, and the remedy is well understood by anyone who has carried a wallet through one. Terrorism is rare and catastrophic. Civil unrest, in the Belgian case, mostly means demonstrations announced in advance that begin and end at published times. Collapsing the three into a single grade is what makes the notice read more heavily than the underlying risk warrants.
The evidence behind it is real and mostly historical. The attacks on Brussels Airport and the metro in March 2016 killed 32 people. An attack in Brussels in October 2023 killed two Swedish nationals. Both are matters of record, and both are why the security posture around Belgian transport looks the way it does today.

