https://en.wikipedia.org/wiki/War_of_1812#Unpreparedness

Apparently having a very small army didn't help when Jefferson's party wanted a war. I guess they over-rated state militias?

A lot of the founders' ideas didn't work out they way they hoped. (For example, they tried to prevent political parties and failed utterly.)

When people quote American founders out of context, it's as if it settles things, but they were just making it up at the time, often based just on what sounds good. Jefferson in particular was often a rather impractical man, but a popular politician who promoted a lot of bad ideas (and a few good ones) that made for good politics at the time. He's hardly the only one.