Complex sentence: Molotov was unable to intercede on Browder's behalf to reintegrate him into an American Communist Party which had by now come to regard him as an undisciplined opportunist and unreliable leader.
Split into simpler sentences: Molotov was unable to intercede on Browder's behalf to reintegrate him into an American Communist Party. By then its leaders regarded him as an undisciplined opportunist and unreliable leader.