Some ideas:
- Rosas was, according to some historians, preparing to go to war agianst Brazil. In 1850, he controled all the country and an ally of him ruled over all of Uruguay except Montevideo (which had been under siege for 7 years). But Urquiza, governor of Entre Ríos, deserted him, and, toghether with Brazil, invaded Uruguay. Then, Brazil, Uruguay, Entre Ríos and the Argentine exiles living in Montevideo attacked Rosas, beating him in Caseros. Paraguay was neutral.
- Had Urquiza not betray him (in his eyes) he might have pose a serious challenge to Brazil. The most he could have hoped, however, is to have Brazil recognize Argentine soveraignty over Uruguay. I don't know enough about Brasil in 1850-51 to know if he could have achieved so.
- If Urquiza deserts him and everything goes as in OTL till the battle of Caseros (February 1852), Rosas might have still win. The battle was a close thing. But the victory would only mean Rosas stays in power a bit longer. It all depends, however, on what Brazil does. Would they raise and send another army? Even if their Argentine and Uruguayan allies abandon the fight? A victory in Caseros might mean either that Rosas falls just a few months later, or (if Brazil gives up) that he stays in power indefinetedly (or untill he pisses of somebody bigger than him willing to fight him until he's oult of power (something that nor Great Britain nor France, who also had problems with him in the past, weren't willing)
- San Martín died in 1850, and was in France, and to old to play a decissive roll. He could have intervened in the civil wars in the late 1820ies, but he didn't. Who would he have supported had he intervened (federalist or unitarians) is an issue that is the matter of heated discussions among local history fans.