Daniel Walker Howe on the US/Mexican balance of power at the dawn of the war:
The United States census of 1840 counted a burgeoning population of 17 million. The population of Mexico, by contrast, had declined by 10 percent during the prolonged disorders of her revolution against Spain and then leveled off; the government’s calculation of 1842 (not an actual enumeration) showed 7 million Mexicans. The economies of the two countries displayed an even greater inequality. The years since 1815 had not been kind to the former New Spain; plagued by political instability, independent Mexico had not realized the economic potential of her natural resources. Fierce localism and poor transportation hindered the emergence of an integrated nationwide economy even more than they did in the United States. Independent Mexico received little European immigration and had even expelled its Spanish-born people, losing their talents and skills. Mexico’s gross national product fell to less than half the peak attained in 1805; not until the 1870s did it exceed that level.
That’s a reminder, among other things, that there’s a strong nationalist case for making the country more open to immigration. Had the pre-WWI United States not had a basically open borders policy, we’d still be a very rich country, but probably not a “great” one. Instead we’d be geographically smaller, less densely populated, and much more agriculturally oriented — more like Australia perhaps.