Is 90 minutes enough for an international connection?

For domestic-to-international: usually yes. You're already through security and just need to get to the international gate. Most airports have 60-90 minute MCTs for this direction.

For international-to-domestic: it's risky. When arriving internationally, you must go through customs and immigration (CBP), collect your checked bags, re-check them, and then go through TSA security again. At efficient ports of entry, this takes 45-60 minutes. At congested ones during peak hours, it can easily take 90+ minutes.

The worst airports for international connections: MIA and JFK regularly see CBP wait times of 60-90 minutes during peak arrival banks (2-5 PM when European flights land). LAX is similar. The best: smaller international gateways like MSP, DTW, and SEA process customs in 20-30 minutes. For a safe international connection, budget 2.5-3 hours at major gateways and 2 hours at smaller ones. If the airline sold you the connection on one ticket, they're on the hook to rebook you if you miss it — but that doesn't save your schedule.