So Explain Why Iran Will Sign a Nuclear Agreement

The Obama administration says that a nuclear agreement with Iran is the only way to avoid war.

I assume this means that the repercussions of Iran getting nuclear weapons is so bad that we would have to attack Iran to prevent them from getting nuclear weapons.

So my question is, if it is worth going to war with Iran to keep them from getting nukes, is it worth avoiding a bad agreement to keep Iran from going nuclear?

Because the administration seems to assume that the only choices are war or whatever deal that Iran (who wants nukes) will accept.

Isn't there a major flaw in the administration's logic?