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First French advance

Foreign Minister Manuel Doblado invited the commissioners to travel to Orizaba and there the three powers proceeded to officially recognized the government of Juárez along with Mexican sovereignty. On 9 April 1862, agreements at Orizaba between the allies broke down, as France made it increasingly clear that it intended to violate Mexican sovereignty in violation of previous agreements. The British informed the Mexican government that they now intended to exit the country, and an arrangement was made with the British government to settle its claims. Spain also agreed to evacuate the country.

Minister Doblado on April 11, 1862, made it known to the French government that its intentions were leading to war. Armed conflict finally broke out as French forces attempted to head for Mexico City. On 5 May 1862, Mexican forces commanded by Ignacio Zaragoza and future president of Mexico, Porfirio Díaz repulsed the French at the Battle of Puebla while the latter were trying to ascend the hill towards the fortified positions of the city. The French retreated to Orizaba to await reinforcements. On October 27, 1862, congress granted President Juárez emergency powers to meet the needs of the ongoing invasion.

Meanwhile the French increasingly gained the collaboration of Conservative Party generals who remained in the Mexican countryside in the wake of the Reform War. Monarchism had died out in Mexico by the time the French intervention began and the Conservative Party was initially reluctant to join the French in establishing a monarchy. The Spanish General Juan Prim, who had been part of the joint expedition would report to his government that there had been no monarchists in Mexico. The Conservatives would eventually be won over as they opportunistically sought the military aid to return themselves to power after their loss in the Reform War.