Saruman tries to master the ring, but he doesn't completely achieve such mastery. The Ring still wants to be with its true master.
Sauron, immediately realizing that his erstwhile ally has the ring, immediately mobilizes forces from Dol Guldor and Mordor to lay seige to Isengard. Isengard eventually falls. Sauron reclaims the ring and rules Middle Earth with an iron fist that would even make Morgoth smile.
Now, if the Free Peoples attempt to attack Sauron's forces as they lay seige to Isengard, they might be able to strike both foes--Sauron and Saruman--a crippling blow. Even if they defeated both of these armies, however, they would still be left with the Ring and the original quest unfulfilled. More than likely Denethor or Theoden would claim the ring if their forces took part in the battle, and that would bold ill for the Free Peoples as well....