But the merriment soon subsided, as Brazilians and Argentinos came to realize what their peril might be from a huge army of skilled and valiant soldiers, a veritable horde of fighting fanatics, drawn up in a compact little land, centrally located and affording in other respects every kind of strategic advantage.
When Brazil invaded Uruguay and restored the Colorados to power, Lopez demanded permission from Argentina to cross its frontier, for the purpose of assailing his enemy from another quarter.

When the permission was denied, Lopez declared war on Argentina also.

It was in every respect a daring step, but Lopez knew that Argentina was not so well prepared as his own state for a war of endurance.

Uruguay then entered into an alliance in 1865 with its two big "protectors." In accordance with its terms, the allies agreed not to conclude peace until Lopez had been overthrown, heavy indemnities had been exacted of Paraguay, its fortifications demolished, its army disbanded, and the country forced to accept any boundaries that the victors might see fit to impose.
Into the details of the campaigns in the frightful conflict that ensued it is not necessary to enter.

Although, in 1866, the allies had assembled an army of some fifty thousand men, Lopez continued taking the offensive until, as the number and determination of his adversaries increased, he was compelled to retreat into his own country.