During the months of July and August, General Obregon was preparing for his advance South. He had little time to waste, for even before he was through with his preparations, he was forced to advance on San Blas, Sinaloa. A strong detachment of Federals were sent up from Mexico City to reinforce the defeated Huerta troops who landed at Topolobampo. But General Obregon was not taken by surprise. Having assigned Generals Hill and Iturbe to proceed against the Federals, he himself continued his march further south. His objective point was the city of Sinaloa. In the meantime Generals Hill and Iturbe had succeeded in defeating the Federal troops which landed in Topolobampo, and joined General Obregon in his attack upon the city of Sinaloa. The storming of Sinaloa was one of the fiercest battles of the entire campaign. It lasted nearly five days and again, as in all the previous battles, the Federals retreated so hastily that they did not have time to take their guns and ammunition with them. A great quantity of ammunition and provisions were left behind by them, of which the Constitutionalists were much in need. One of the great difficulties which constantly confronted General Obregon was the guarding of the frontier and the positions all along the coast. The slightest error in the guarding of those positions might have caused the annihilation of his entire army by a rear attack. So that, whenever he took a city from the Federals, he was confronted with the question of protecting that point. He was forced to always leave troops behind him, to guard those conquered cities. Had he not done so, the Federals might have sent up new forces by way of the Pacific and re-taken the conquered posts. The most important of all the battles of the entire campaign was the storming of Culiacan. The Federals, realizing the dangers of Obregon’s swift march, massed a strong force of troops in that city, numbering about seven thousand. Needless to say, they were much better equipped than the Constitutionals, who always had more men than rifles and guns. When General Obregon, who personally conducted the battle, reached the city, the Federals were well fortified in their positions. The fight lasted a whole week, and fighting continued day and night, almost without cessation. At the end of that time, the Federals were badly beaten and were forced to retire to Mazatlan. The taking of Culiacan meant to the Constitutionalists more than just an ordinary victory. It meant the success of the operations towards their goal, and the weakening and disintegration of the Huerta troops. The winning of this battle enabled the Constitutional forces to move further south to the Territory of Tepic, where General Obregon took the city of Acaponeta and San Blas by storm.