Complex sentence: After a trip to Europe in 1959, overwhelmed by the quality of art he saw there (especially that of Georges Braque, whom Tate briefly visited) Tate insisted on borrowing or buying back his paintings from their owners, so he could "improve" them.
Split into simpler sentences: After a trip to Europe in 1959, he became overwhelmed by the quality of art he saw there (especially that of Georges Braque, whom Tate briefly visited). On his return to America, Tate insisted on borrowing or buying back his paintings from their owners, so he could "improve" them.