In 1586, James signed the Treaty of Berwick with England. That and his mother's execution in 1587, which he denounced as a "preposterous and strange procedure", helped clear the way for his succession south of the border. Queen Elizabeth was unmarried and childless, and James was her most likely successor. Securing the English succession became a cornerstone of his policy. During the Spanish Armada crisis of 1588, he assured Elizabeth of his support as "your natural son and compatriot of your country".