50 years ago today, the Tet Offensive began.

Everyone knew it was coming. At the U.S. Tactical Operation Center in Saigon, they had a betting pool going as to when the attacks would start.

But it turns out that–as at Pearl Harbor–the knowledge that the enemy is about to attack is, by itself, actually completely useless. What you need to know is when, where and how. If you don’t have those specifics, even if you know something is coming, you can still be caught completely off guard.

Which is exactly what happened.