Obregón instigated land reform and strengthened the power of organized labor. He gained recognition from the United States and took steps to settle claims with companies and individuals that lost property during the Revolution. He imposed his fellow former Sonoran revolutionary general, Calles, as his successor, prompting an unsuccessful military revolt. As president, Calles provoked a major conflict with the Catholic Church and Catholic guerrilla armies when he strictly enforced anticlerical articles of the 1917 Constitution. The Church-State conflict was mediated and ended with the aid of the U.S. Ambassador to Mexico and ended with an agreement between the parties in conflict, by means of which the respective fields of action were defined. Although the constitution prohibited reelection of the president, Obregón wished to run again and the constitution was amended to allow non-consecutive re-election.  Obregón won the 1928 elections, but was assassinated by a Catholic zealot, causing a political crisis of succession. Calles could not become president again, since he has just ended his term.  He sought to set up a structure to manage presidential succession, founding the party that was to dominate Mexico until the late twentieth century. Calles declared that the Revolution had moved from ''caudillismo'' (rule by strongmen) to the ''era institucional'' (institutional era). 