Complex sentence: While Leisler claimed to have acted to support the Protestant accession against Jacobite officeholders in New York, he was arrested by the newly appointed governor of New York in March 1791, condemned and executed in New York City for treason against the English monarchs William and Mary.
Split into simpler sentences: While Leisler claimed to have acted to support the Protestant accession against Jacobite officeholders in New York, he was arrested by the newly appointed governor of New York in March 1791. With opponents active against him, he was condemned and executed in New York City for treason against the English monarchs William and Mary.