At last on the morning of April 9th, Walker rode out of Granada at the head of five hundred men, four-fifths of them Americans, and pressed rapidly southward towards Rivas, where Mora lay encamped with Prussian von Bulow and three thousand regulars. There were several English, French, and Germans acting with the Costa Ricans, some as volunteers and many as mercenaries. At eight o'clock, on the morning of the 11th, Walker's forces entered Rivas in four detachments by as many different routes. The order of battle was that of a simultaneous assault, the several detachments to unite at the centre of the town. It was faithfully carried out, although the Costa Ricans, soon recovering from their surprise, behaved gallantly, using their firearms with precision and coolness, and picking off the American leaders with fatal accuracy. The combat lasted through four hours. At its termination Walker had gained possession of the plaza and cathedral, but at a cost of fifty killed and wounded. About two hundred of the enemy were killed and twice as many wounded. They were receiving reinforcements, but did not venture from behind their adobe walls to renew the contest. Setting fire to the houses near the plaza, they kept up a desultory sharp-shooting from the adjacent buildings. The Americans improvised a temporary hospital within the cathedral, whence at daybreak the wounded were deported, well guarded by their comrades. Mora did not oppose their departure, being well content to be rid of his troublesome visitors. Walker's loss in officers was severe. Early in the fight Colonel Machado, commanding the native soldiers, fell mortally wounded. Five captains and six lieutenants also perished, and there were twelve other officers among the wounded. Of Walker's staff Captain Sutter alone survived. This mortality was due not more to the marksmanship of the enemy than to the reckless courage of the victims, who made it a point of honour to volunteer for every desperate adventure. Ten of them at one time had charged, armed only with revolvers, on a barricade, whence they dislodged over a hundred of the enemy's riflemen. By this time the aspect of affairs had changed materially, and the situation of the invading army had become extremely perilous. The Legitimists, whom Mora had expected to unite with him in expelling the American usurpers, he found to be few and faint of heart, while the wanton insolence of his own men had tended to alienate whatever of sympathy they might have found among the poorer classes. In a word, the repulse of Walker at Rivas, if that can be called a repulse which was an unhindered withdrawal, was to Mora the signal of defeat. Unable to conquer an enemy of one-sixth his strength, and not daring to lessen his odds in the hazard of a pitched battle--much less in a siege of Granada--he lay at Rivas exhausted and impotent. It needed but one other enemy to complete his overthrow. That enemy, always a potent one beneath the tropic sun, appeared.