A rare nationwide ground stop has halted almost all US flights, and it's bad news for travelers and airlines while raising fresh fears about war with Iran.

• The FAA grounded nearly all flights after a communications breakdown hit air traffic control centers across the country, with thousands of flights delayed and hundreds already cancelled.
• This is only the third time this has ever happened, after 9/11 and a 2023 outage, and officials say it is not confirmed to be a cyberattack, though the FAA is still checking.
• A defense contractor that runs key radio networks for air traffic control is reportedly linked to the outage, possibly tied to a software update gone wrong.
• The timing is raising eyebrows because it hit right after a drone scare shut down a UK air base and while Israel's spy chief is in Washington pushing for the US to rejoin the fight against Iran.
• Europe and Mexico have paused flights into the US, and officials have moved Treasury Secretary Bessent to a military base over security concerns tied to the Iran tensions.

Outlook: Airlines expect the backlog to take days to clear, and the cause of the outage should become clearer as investigators rule out or confirm a cyberattack.