I was thinking the other day, as one sometimes does, about that much over examined favourite of Alternative History discussions a Trent Affair war. Now frankly the war itself has been done to death here and else where but it did occur to me that the impact of the war on Asia has not been discussed and it could be huge.
Consider a a war in which the French make the most of the opportunity presented to them to secure Mexico for their empire. In such a scenario the USA would have no presence in Asia and that of the French and British (east of Burma anyway) are likely to be deployed to Mexico and California respectively. This could mean that
1) The Ti Ping rebellion does not end as swiftly as it did in OTL due to lack of support for the Imperials by Britain and France. The war could linger on for several more years, the heavenly kingdom might not even fall at all.
2) Similarly the Chinese Imperials might take the opportunity presented by British and French weakness to renage on the unequal treaties imposed that year at the conclusion of the Second Opium war.
3) The French are unlikely to be able to pursue the Cochinchina wars and suppress and occupy parts of Vietnam.
4) Siam gains a breathing space from the predatory ambitions of France and Britain.
5) Without the USA, France and Britain pushing things so hard in Japan things might happen very differently the Prussians might be the greatest benefactors from the opening of Japan.
6) In a bad defeat the USA could very well lose all access to the Pacific to the British, the independent Californians, Confederates, a French Mexico or some combination of the above. Thus negating OTL USA's colonial acquisitions of Guam, Hawaii and the Philippines.