Up until 2004, it was completely normal for US citizens to travel to Canada and Mexico (and back, of course) without a passport.

Reentry theoretically shouldn't require a passport, as long as you can prove citizenship.

I can testify to the process for something similar since I had a passport stolen when I was traveling in Spain. I went into the consulate, told them what happened and they asked me a few questions about my hometown (high school I went to, favorite restaurant, previous address). They noted my answers and had me fill out the normal passport form (including new photos) and assured me it would be ready in just under a week so I wouldn't miss my flight home. It was pretty painless, though I'd bet that just appearing American (accent, skin color, etc) might've helped.

I was talking about entering the country without one. You received a backup one outside of the country.

Well, it's the same authentication problem of validating US citizenship without documentation. And the passport I was issued was in no way a backup, it was valid for the normal 10-year period and, apart from the note that said it was issued at the US embassy in Madrid, identical to one I would have been issued through the normal process inside the US.