The Costa Ricans invade Nicaragua--Second battle of Rivas--The enemy meet a new foe--Rivas orders an election--Walker a candidate--Treason of Rivas--Murder of Estrada--Coalition of the Northern States against Nicaragua--Walker chosen President--Inauguration and recognition by the United States minister--Tradition of the "Gray-eyed Man." Walker was less concerned about his enemies in the United States than those nearer home, though he never committed the mistake of undervaluing a dangerous foe or the weakness of forgiving him. Three thousand Costa Ricans had crossed the border and overrun the southern part of Rivas. It was no time for fever of body or mind. Walker arose from his bed and summoned his forces to strike a vigorous blow for his rights. Rivas, the President, was at Leon, watching and waiting; he had placed the southern departments under martial law, and given absolute power to the commander-in-chief. Walker no longer opposed the enemy's march on Rivas, as his object in holding the Transit had been lost with the withdrawal of the steamers. All the American troops at Rivas and Virgin Bay were accordingly removed to Granada, with the ostensible purpose of retreating at once to Leon. When the enemy entered Virgin Bay they found there only the native inhabitants and a few foreign _employés_ of the Transit Company. Without a word of warning, they opened fire on the latter, killing some nine or ten unarmed servants of Mr. Vanderbilt, and with a zeal for which that gentleman would have been far from grateful, burned all of the company's property in wharves and warehouses which they could find. After completing the work of destruction, they marched to Rivas, where President Mora took up his abode and cautiously awaited the movements of Walker. The latter kept his counsel so well that no one knew whether he intended retreating to Leon or abandoning the country entirely. The latter course seemed the more probable, as the lake steamer, _San Carlos_, had been for some days engaged in carrying men and munition across the lake and down the river to Forts San Carlos and Castillo Viejo. A side light was thrown on these movements, when Lieutenant Green, with only fifteen men, surprised a Costa Rica force of two hundred at the mouth of the Serapiqui, killing twenty-seven of them and putting the rest to flight.