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.