Commenting on these measures, President Buchanan in his annual message to Congress of December 3, 1860, had sounded a note of warning to Europe indicating that American principles would compel the use of force in aid of Mexico if debt-collecting efforts were made the excuse for a plan "to deprive our neighbouring Republic of portions of her territory." But this was at the moment of the break-up of the Union and attracted little attention in the United States.

For the same reason, no longer fearing an American block to these plans, the three European Governments, after their invitation to the United States to join them had been refused, signed a convention, October 31, 1861, to force a payment of debts by Mexico.

They pledged themselves, however, to seek no accession of territory and not to interfere in the internal affairs of Mexico.
In this pledge Great Britain and Spain were sincere.

Napoleon III was not--was indeed pursuing a policy not at first understood even by his Ministers[546].

A joint expedition under the leadership of the Spanish General Prim was despatched, and once in Mexico took possession of customs houses and began to collect duties.