You're basically guaranteed a promo to captain, after which promotion gets a lot harder and rarer.
It's the officer version of getting to E-4 before your enlistment ends -- as long as you're not a moron and get busted down you'll get there.
A Captain rank is in no way a qualification for Sec of Defense when there are literally hundreds of 4-star Generals and Admirals with well established track records of running operations during 20 years of Iraq and Afghanistan.
