Before the war
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United Provinces, the United Provinces didn't liked the idea because the orientals were federals, but at the end it decided to help the province.
The war
The United Provinces took advantage of 
Brazil's instability to attack, these two had a very close fight and a lot of the battles ended up being inconclusive.
On the land battles, the United Provinces won all the main battles (Battle of Sarandí and Battle of Ituzaingó). Argentina was able to advance a lot through Cisplatina, not giving opportunity to the Empire of regaining terrain. But there were a lot of inconclusive battles and never invaded the capital.
On the sea battles the United Provinces and the Brazilian Empire won one of the two main battles, Argie won the Battle of Juncal, while Brazil won the Battle of Monte Santiago. And most of the rest of the battles were inconclusive. Despite the close fights the argentine navy was small and ended up in a bad state after the war.
UKball forced asked for a negociation, and both accepted. The United Provinces gained some land on Misiones and Cisplatinaball became 
Uruguayball against it's own will.
Fun fact: during the Battle of Ituzaingó Argentina theft a marching song about the battle that Brazil was making in case of winning the fight. 
Argentina won and made the march one of its presidential emblems and mocks 
Brazil even to this day by playing the march.