Complex sentence: The federalization of Buenos Aires, that politically separated the city from the Buenos Aires Province to put it under direct control of the national government, was a constant aspiration of the other provinces of Argentina since the formation of the national state.
Split into simpler sentences: The federalization of Buenos Aires politically separated the city from the Buenos Aires Province to put it under direct control of the national government. It was a constant aspiration of the other provinces of Argentina since the formation of the national state.