Complex sentence: France and Great Britain jointly retaliated against Argentine expansionism, blockading Buenos Aires for most of the late 1840s, but were unable to halt Rosas, whose prestige was greatly enhanced by his string of successes.
Split into simpler sentences: France and Great Britain jointly retaliated against Argentine expansionism, blockading Buenos Aires for most of the late 1840s. However, they were unable to halt Rosas, whose prestige was greatly enhanced by his string of successes.