Following the fall of the Berlin Wall and the collapse of European communism, both Republicans and Democrats were eagerly looking at ways to transfer money from the Defense budget to other areas. They called this the "Peace Dividend." According to a joke of the time, they agreed to the following ways to divide up the Peace Dividend:

50% would go to urban renewal and jobs training
50% would go to health care
50% would go to deficit reduction
50% would go to highways and infrastructure
50% would go to tax cuts
...

You get the idea. I was an officer with a reserve commission (1985 year group) at the time. As soon as I saw the Berlin Wall fall, I knew that no matter what kind of job I did, my days in uniform were numbered. At the start of 1992, there were 2000 of us in my career category (1985 year group reserve officers that were not pilots). At the end of the year, there were 105 left, a 94% kill ratio. That was just one career category out of many.

These cuts began under a Republican administration (Bush 41) and went even deeper under Clinton. In 1991, we had 18 Army divisions. We now have somewhere around 10 and many of them are below strength. We've also eliminated many aircraft wings from the Air Force and several aircraft carrier battle groups from the Navy. Several years of neglect and over deployment on "meals on wheels" missions have seriously cut into the training and combat readiness of many military units. They lack the spare parts to keep their equipment operational, they lack the training to do their missions, and until recently they lacked the morale to give a damn. That 3rd deficiency is probably changing but getting training and readiness up to standard will take some time.