The failure of Mexico to find a way adequately to finance the war seriously undermined the country's ability to resist the American onslaught. She entered the war with an empty Treasury, inadequate revenues and weak credit. The lack of money left the government disorganized and the army poorly equipped, seldom paid and badly trained and supplied. Class, geographical and political divisions compounded the problems. The economic elite were prepared passionately to praise the idea of Mexico but not to contribute to its support. Individual and class interest, not national community, were paramount. None of the governments that attempted to deal with the crisis were able to collect sufficient revenues or provide political stability. The story of Mexico's war financing is one of a continuous search for loans.

The lack of records and Treasury reports greatly complicate the task of understanding Mexico's finances during the period 1846–8. Historians attempting to reconstruct Mexico's war effort have relied on known transactions, which provide an understanding of the methods but in no way give a complete picture. The government and army survived on what ordinary revenue flowed into the Treasury, forced loans, requisitions, donations, sale of government property and franchises and loans secured by Church property. The Mexican army supplemented its meagre allowances by drawing supplies from nearby communities or the surrounding countryside and paid by issuing treasury paper that might or might not be honoured.

When the conflict opened much of Mexico's future revenues were pledged to repay existing loans. Barbara Tenenbaum estimates that 4,780,560 pesos of the 1845 income was pledged to foreign and domestic lenders. She places 1845 revenues at 11.7 million pesos. Tenenbaum provides no figures for 1846 but believes tax collections dropped to 8.8 million pesos in 1847 under the pressure of the American blockade and the disruption of domestic economic activity. Wilfred Hardy Callcott reckons Mexico's federal government's revenues at 10,680,000 pesos in 1845 and 10,250,000 in 1846. The ordinary revenues, from 700,000 to 900,000 pesos a month, in no way sufficed to maintain large armies.