Why was the Kellogg-Briand Pact considered useless?

There were two main issues with the Kellogg-Briand Pact that caused it to have little effect.

1. There was no way to enforce the pact, or punish those who disobeyed.
2. "Self-defense" was never fully defined, so there were many ways around the pact's terms.

These weaknesses were evidenced by the Japanese invasion of Manchuria (where no action was taken despite Japan's violation of the pact), which was a great failure for the Kellogg-Briand Pact.