Complex sentence: Saionji, when he could, chose as prime minister the president of the majority party in the Diet, but his power was always constrained by the necessity of at least the tacit consent of the army and navy.
Split into simpler sentences: Saionji, when he could, chose as prime minister the president of the majority party in the Diet. But his power was always constrained by the necessity of at least the tacit consent of the army and navy.