López now strikes at his enemy's main…

López now strikes at his enemy's main force in Uruguay; he is, however unaware that Argentina has acquiesced to Brazil's Uruguay policy and will not support Paraguay against Brazil.

The invasion of Brazil’s Rio Grande do Sul province is the second phase of the Paraguayan offensive.

To raise the support of the Uruguayan Blancos, the Paraguayan army has to travel through Argentine territory.

In March 1865, López had asked the Argentine government's permission for an army of twenty-five thousand men, led by General Wenceslao Robles, to travel through the province of Corrientes.

The Argentine President—Bartolomé Mitre, an ally of Brazil in the intervention in Uruguay—had refused.

A hastily summoned congress composed of López's own nominees had bestowed the title of Marshal upon him and given him extraordinary war powers.

On April 13, 1865, Paraguay declares war on Argentina.

A Paraguayan squadron, coming down the Río Paraná, traps two Argentine war vessels in the Bay of Corrientes.

Immediately, General Robles' troops take the city.

López institutes a provisional government of his Argentine partisans, and announces that Paraguay has annexed Corrientes Province and Entre Ríos Province.

The Paraguayans advance approximately two hundred kilometers (1one hundred and twenty miles) south before ultimately ending the offensive in failure.

By invading Corrientes, López is attempting to obtain the support of the powerful Argentine caudillo Justo José de Urquiza, governor of the provinces of Corrientes and Entre Ríos, and the chief federalist hostile to Mitre and to the government of Buenos Aires.