It is questionable whether or not there were armies of Balrogs. See
The Witch King and Gothmog,
The Seven Thanes of Morgoth, and
Bye Bye Balrogs. Obloquy and I have a good debate in the first two threads about the number of Balrogs in existance. There might have been as few as 3 Balrogs in existance. Personally, I'm of the opinion that there were more like 15-20 Balrogs, with several being killed in the Battle of the Powers. Sauron did not put the Balrogs where they are located, the Balrogs are Maiar with their own wills, created by Illuvitar, and corrupted by Melkor. Any Balrogs that were left in the world at the time of the Third Age (it is possible that Durin's Bane was the only Balrog to survive, even if I don't believe that) fled during The War of Wrath (the battle where Melkor was overthrown). The Balrog of Moria and Sauron at least had an understanding, considering that Sauron's orcs had been cohabiting with the Balrog for several hundred years. Without the Ring I do not think that the Sauron would have been able to control the Balrog. However, with the Ring Sauron probably would have been able to use the Balrog to attack Lorien or perhaps Rivendell (Sauron was deprived of Smaug; in the appendices it hints that Sauron might have used Smaug to do terrible damage to Eriador and Rivendell). The power of Balrogs was not in assaulting armies, but in fighting beings of great power, like elf lords or other Maiar (Gandalf).
[ March 02, 2002: Message edited by: Thingol ]