Newly elected President Benito Juarez issued a moratorium on July 17, 1861, to help get a handle on his country's wrecked economy, according to UCLA's Chicano and Latino issues resource center.
The moratorium stipulated a hold on all foreign debt payments for the next two years so that Mexico could get out of financial ruin. Payments could resume after the two-year mark, but in the meantime Mexico was forced to default on debts abroad.
England, Spain and France - all of which Mexico owed money to - were furious. According to History.com, all three sent naval ships to Veracruz to demand reimbursement. British and Spanish forces eventually negotiated with Mexico and withdrew, but it was France that decided to take severe action.
Seeing an opportunity to take advantage of a fallen nation, French ruler Napoleon III had hoped to be victorious over the weakened Mexican army and carve out an independent empire for France.
According to UCLA, there is some speculation that the United States' enactment of the Monroe Doctrine in 1823, which stated that any European attempts to re-colonize any part of the Americas would be considered an act of war, may have sparked the French invasion frenzy. At the time, the United States' quick and immense expansion was seen as a threat to other world powers.