How important is War College for promotion to Colonel?

The promotion board instructions change each year, but the standard is usually you must be at least ENROLLED the War College to be considered competitive for promotion to colonel. The board is instructed as to what the Army needs. Although excellent performance as a LTC will sometimes results in a promotion to Colonel without AWC, the active duty promotion rate to Colonel was only 41% in 2015. That's 41% of the officers who were eligible, a smaller group than the total number of the LTCs. Although it is possible to be promoted without AWC, it is less and less likely.

I would always turn down LTCs who were seeking an MRD extension who had not applied to the War College, explaining to them that when they decided not to apply they had purposefully decided that their career would end at Lieutenant Colonel and 28 years.

There is an old saying that you need EITHER War College or Battalion Command to make O-6 and I have found this generally to be true. I have had a few friends make it without either, but they were very, very good and almost certainly had top block OERs. I would dare to say that most are promoted to Colonel BEFORE they have completed War College. That was certainly true in my case. So to your answer, no, it is not essential - but it helps!

In the AF, we do first cut by making three piles, and lack of PME is almost always a self-eliminator. The Army may prioritize differently, but when you have an optional task like AWC that some choose to do and some don't, all other things being about equal, I choose the man or woman who decided to grin and bear it and complete the course. It is one of the few objective criteria, other than command, and if you haven't deployed every year since you became eligible for AWC, the board may start right off by putting you in the gray pile.