The United States had at the moment but fourteen thousand men in the islands, under the command of General Otis. Some of these were volunteers who had been organized to fight Spain and who could not be held after the ratification of peace. Congress had, indeed, provided for an increase in the regular army, but not sufficient to provide the "40,000 effectives for the field," whom Otis had requested in August, 1899. There were, of course, plenty of men available in America for service in the Philippines, and finally twelve regiments of volunteers were raised, two of which were composed of negroes. Aguinaldo's strength lay in the configuration of the country, in its climate, which for four centuries had prevented a complete conquest by the Spaniards, and in the uncertainty which he knew existed as to how far the American people would support a war waged apparently for conquest, against the wishes of the Filipinos. On the other hand, the chief advantages of the American forces lay in Aguinaldo's lack of arms and in the power of the American Navy, which confined the fighting for the most part to Luzon. In March, General MacArthur began to move to the north, and on the last day of that month he entered Malolos. On the 23d of April he pushed farther northward toward Calumpit, where the Filipino generalissimo, Luna, had prepared a position which he declared to be impregnable. This brief campaign added a new favorite to the American roll of honor, for it was here that Colonel Funston, at the head of his gallant Kansans, crossed the rivers Bag-bag and Rio Grande, under circumstances that gave the individual American soldier a prestige in the eyes of the Filipinos and a reputation which often ran far ahead of the army. General Luna had torn up the ties and rails of the steel railroad bridge over the Bag-bag, and had let down the span next the far bank. Thus cut off from attack by a deep river two hundred feet wide, the Filipino commander had entrenched his forces on the farther side. Shielded by fields of young corn and bamboo thickets, the Americans approached the bank of the river. A naval gun on an armored train bombarded the Filipinos but could not silence their trenches. It was therefore necessary to cross an the bridge, and under fire. General Wheaton ordered Colonel Funston to seize the bridge. With about ten men Funston rushed the nearer end which stood in the open. Working themselves along the girders, the men finally reached the broken span. Beyond that, swimming was the only method of reaching the goal. Leaving their guns behind them, Colonel Funston and three others swung themselves off the bridge and into the stream. Quite unarmed, the four landed and rushed the nearest trenches. Fortunately these had been abandoned under American fire, and rifles and cartridges had been left behind. Thus this aquatic charge by unarmed men secured the bridge and enabled the American troops to cross.