Complex sentence: On 11 February 1387, John I married Philippa of Lancaster, daughter of John of Gaunt, who had proved to be a worthy ally, consolidating the Anglo-Portuguese Alliance that endures to the present day.
Split into simpler sentences: On 11 February 1387, John I married Philippa of Lancaster, daughter of John of Gaunt, who had proved to be a worthy ally. The marriage consolidated an Anglo-Portuguese Alliance that endures to the present day.