If Iran retaliates by, say, launching a cyber attack on the US National Grid, shuts down the Eastern Seaboard and people die - that will be considered an attack against NATO. Likewise, if Iran launches assassinations of US military personnel in Europe that will be considered an attack against NATO.
If the US invokes Article Five then the European nations will have no choice but to go along because without the US we cannot collectively oppose Russia - certain key members are not meeting their basic commitments, and others are about to be kicked out.
Hence "no choice".
This is completely different to the EU, which is a political project that the majority of voters rejected - i.e. federalisation.
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