The Mexican Revolution was a major revolution that was not a unified struggle, but an extended sequence of armed regional conflicts. It transformed Mexican culture and government through the writing of a new constitution. Although the regime of President Porfirio Díaz was increasingly unpopular after 31 years, there was no foreboding that a revolution was about to break out in 1910. The regime failed to find a controlled solution to the issue of presidential succession, resulting in a power struggle among competing elites, and elites and the middle classes that sometimes involved the "masses". This provided the opportunity in some places for agrarian insurrection, most prominently in Morelos under Emiliano Zapata. Although the conflict was primarily a civil war, foreign powers, having important economic and strategic interests in Mexico, figured in the outcome of Mexico's power struggles. The United States played an especially significant role. Northern Mexico, with access to arms from the U.S., saw the most military clashes. With the exception of Zapata, the revolutionary leaders of first rank were all from northern Mexico, Francisco I.