Complex sentence: Despite his successes, Mundhir was accused by Maurice of treason during the preceding campaign, namely of having revealed the Byzantine plan to the Persians, who then proceeded to destroy the bridge over the Euphrates.
Split into simpler sentences: Despite his successes, Mundhir was accused by Maurice of treason during the preceding campaign. Maurice claimed that Mundhir had revealed the Byzantine plan to the Persians, who then proceeded to destroy the bridge over the Euphrates.