The year of 1865 was of absolute disaster for the Mexican Republicans. In August 15, 1865 at the siege of Chihuahua, Colonel Augustine Henri Brincourt along with 600 soldiers were laying seige to the city of Chihuahua and after 10 days the city surrendered. But it was who was in the city that made it the ultimate prize: President Benito Juarez.

Colonel Agustin Henri Brincourt

President Benito Juarez captured
The Colonel was shocked, he now had the head of the Republic cause in his hands so he wasted no time in sending Juarez back to Mexico City, heavily guarded of course, and once Juarez reached Mexico City he now knew there was no way he was going to get out of this and so he was utterly devastated but he was still received with great respect and dignity by Emperor Maximilian (who greatly respected him.)

His Imperial Majesty, Emperor Maximilian
Juarez was given a choice: Either accept the amnesty given to him by the Emperor and join his government or accept going into exile. To Juarez, it was no contest, he was a man of principal and he could never compromise with the government who had overthrown him and so on August 29, Benito Juarez was sent into exile in Modena, Italy where he would die, a defeated man in 1872.
The effects of the decapitation of the Republican snake was immediate with the Republicans splintering into multiple groups each battling for dominance, the only man with the most legitimacy was now President Sebastian Lerdo de Tejada and his greatest ally General Mariano Escobedo. Lerdo de Tejada was Minister of Justice and a close friend of Juarez, General Escobedo was by far the strongest of all the Republicans with him being a talented and diligent general along with his strong Northern army of 50,000 soldiers along with support in weapons by the United States, he would be the greatest challenge to the Imperials

President Sebastian Lerdo de Tejada

General Mariano Escobedo
In the United States, President Andrew Johnson was devastated by the defeat and exile of Juarez and immediately gave "morale" support to Lerdo de Tejada, however The U.S. was essentially powerless to do anything to help the Republicans, the US had basically finished a gruelling civil war which end the lives of 600,000 to 1,000,000 Americans and was now in the process of Reconstruction, and was now in a impass; even with President Johnson support, Congress knew his position was unsecured (not to mention they don't care what Johnson says or tries to pass and will block anything he tries to pass.) And the truth was the US was exhausted and really wasn't in favor of going to war with both Imperial France and Mexico, (even if the US intervenes both sides of Mexico would attack the US soldiers.) So the Republicans were essentially alone except with monetary, weaponry, and political support.

President Andrew Johnson