[Kuruharan]

Finwe has already given you part of the answer.

The second half is that all Sauron's power in the War of the Ring was still based on the power of the Ring. As long as the Ring was in existence Sauron could indirectly draw on it's power, since it was a part of him. So, if it is destroyed he does not remain at the same level of strength.

The crumbling of his armies with the destruction of the Ring was keyed to his crippling as a being. His masses of minions required his evil hand in their hearts to provide them with a will. This is another place where the Ring aided his strength. However, when the Ring was destroyed and Sauron crippled, then his will was removed from his minions and they were bereft of purpose and became nothing more than beasts.

However, his armies are largely irrelevant. Once his power was broken, there was nothing more that he could do with them anyway.