Complex sentence: When the two armies met, large portions of Lepidus's forces joined up with Antony, leading Lepidus to negotiate an agreement with him, while claiming to the Senate that he had no choice.
Split into simpler sentences: When the two armies met, large portions of Lepidus's forces joined up with Antony. Lepidus negotiated an agreement with him, while claiming to the Senate that he had no choice.