The war in Iran was the final nail in the coffin. But they were running out of cash for the past few years. If the Iran situation was so bad by itself, we would surely see other airlines failing now.

Spirit could simply be the first of several; the effects may also be delayed. WGA isn't looking good either, for a number of reasons.

Just because it's the first doesn't mean it will be the only one. It goes without saying but apparently you need to be told that there has to a first here, after all. The war is only 2 months in. Full clarity won't come for 2-3 years. It's likely several airlines will take hits on their balance sheets from this that they won't be able to recover from, but they'll fight or go into hardcore refinance mode or get bailed out before actually going bankrupt, but this will remain the ultimate cause.