SIR, -- I have the honor to report, that at the request of Commodore R. F. Stockton (who in September last assumed the titled of Governor of California), I consented to take command of an expedition to this place – capital of the country – and that on the 29th of December, I left San Diego with about five hundred men, consisting of sixty dismounted dragoons, under Captain Turner ; fifty California volunteers, and the remainder of marines and sailors, with a battery of artillery. Lieutenant Emory, topographical engineers, acted as assistant adjutant-general. Commodore Stockton accompanied us. We proceeded on our route without seeing the enemy till the 8th instant, when they showed themselves in full force of six hundred mounted men, with four pieces of artillery, under their Governor Flores, occupying the heights in front of us, which commanded the crossing of the river San Gabriel, and they ready to oppose our further progress. The necessary disposition of our troops was immediately made, by covering our front with a strong party of skirmishers, placing our wagons and baggage train in rear of them, and protecting the flanks and rear with the remainder of the command. We then proceeded, forded the river, carried the heights, and drove the enemy from them after an action of about one and a half hours, during which they made a charge upon our left flank, which was repulsed ; soon after which, they retreated and left us in possession of the field, on which we encamped that night.<br>