One thing I read recently said one of the aims of the maginot line was to delay the Germans (it did -- they had to go through NL/BE), and another was to ensure they went through BE and thus brought the British in to the fight (due to a UK/BE defence guarantee). It did that too.

I think the British also guaranteed Poland, so they we're already in in September '39

But when the line was conceived and built, they couldn't have known Germany would invade Poland first.

Wasn't that that "Quiet war" ie, both france and great britain had guaranteed poland, and declared war, but didn't actually push out troops or really do anything until france was invaded?

Phoney War. War was declared, but no real fighting was going on:

https://en.wikipedia.org/wiki/Phoney_War