Maximilian I was a Habsburg, whose (Catholic!) house had been forced to concede Lombardy-Venetia to Italy (through France) in the treaty of Zurich. Part of this negotiation was understood to be compensated by lands elsewhere. As it happened, the conservative Monarchists in Mexico were looking for a 'legitimate' ruler to solidify their own position against the Liberal Juarez government. When Mexico began to default on debt payments to European governments, they took military action. France's particularly aggressive agenda worried the other two debtholders, Britain and Spain to the degree that they withdrew most involvement with the enterprise.