Agnes stayed around Washington when she met Prince Felix Salm-Salm, a soldier of fortune, and a distant relation to the Emperor Franz Joseph of Austria-Hungary.

When they arrived in Mexico City, the French troops under François Achille Bazaine were leaving, as mounting diplomatic pressure from the United States persuaded Napoleon III to conclude his Mexican venture.

Hence, the couple knowingly embarked on a risky enterprise, joining the weakened and unpopular imperial army.

Simultaneously, the imperial reinforcements that were expected to break the siege were themselves being besieged in Puebla by the republican troops commanded by General Porfirio Díaz and Maximilian was forced to surrender.

Despite last minute efforts by Felix Salm-Salm and a company of Hussars to rescue him, Maximilian was made prisoner on May 15 and a court martial sentenced him to death.

Agnes made great efforts to spare the life of Maximilian and her husband, traveling several times from Querétaro to Mexico City and San Luis Potosí where she held interviews with President Benito Juárez, General Mariano Escobedo, General Porfirio Díaz and several other prominent officers of the Republican Army.

The President's answer has become famous in Mexican history textbooks: "It causes me great pain, Madame, to see you like that on your knees; but even if every king and queen were in your place, I couldn't spare his life.

Agnes agreed, believing he would go through with it, but the escape plan fell through and Maximilian was executed 19 June 1867 in Santiago de Querétaro[5][8] .

Felix Salm-Salm was released from custody in December 1867 and soon after, he returned to Europe, where Agnes joined him, and re-entered the Prussian Army.

Afterward, his widow remained an active seeker for justice in the world, collecting funds for military hospitals.

[citation needed] She lived for several years in Switzerland and Italy, part of the time with her friends, the Baron and Baroness von Stein.