Whoa whoa whoa!
There's an awful lot of misinformation going around this thread!
First, there is no, and there never has been, an E10 the highest paygrade is E9. Senior enlisted advisers to the Secretary of the [service] (the SMA, MCPON, etc) are still E9s. They DO get extra pay while they are in the senior enlisted adviser position, but as the pay is for a duty position, not a promotion in rank, it is treated by the military as a stipend, not a promotion. After their tour of duty as the senior adviser they can and do return to normal E9 positions.
Second, O10 is a rank that is currently filled by 38 officers. These are the four star generals and admirals of the US military. The confusion being caused is by the existence of the rank of General of the Army, aka five star general. THIS is the rank that is only used in wartime and it too is an O10. This rank is reserved for periods of major war. I'll explain why. For star generals are considered leaders of the largest groups, such as Major Army Commands or Army Groups. The need for a five star only arises when the scale of operations becomes so large that multiple Armies are fighting side by side which would have officers of equal rank commanding the operation. The purpose of the General of the Army (or Admiral of the Navy or General of the Air Force) is to bring the officers under one Supreme Commander who will run the operation.
