The 1861 agreement involving Britain, France, and Spain over Mexico’s unpaid debts was the Convention of London.
Signed on October 31, 1861, the convention allowed the three European powers to coordinate an expedition to pressure Mexico’s government. Mexican President Benito Juárez had suspended payments on foreign debts because the country was recovering from years of civil war.
The allied forces landed at Veracruz in early 1862. Britain and Spain negotiated with Mexico and withdrew after reaching an understanding, but France continued its campaign because Napoleon III had broader political ambitions for Mexico.
The Convention of London therefore forms part of the background to the French intervention, but it was not a declaration creating the later Mexican Empire. Cinco de Mayo commemorates Mexico’s victory over the continuing French expedition at Puebla in 1862.