I've reached 1950 in my Civ V game, and it's been an interesting century.
The first war with Siam ended in stalemate; Siam had more troops but I had strong forts that they couldn't break. We made peace, and I fulfilled my pledge with Carthage to attack Brazil the following turn, to punish them for stealing technologies.
Brazil was a much weaker foe, with only three discovered cities, and two of them fell fairly quickly.
Then, as I was advancing on Rio, Siam declared war on me again.
I was forced to make peace with Brazil, as Siam hadn't lost many troops in the last war and had even more, and now that I'd founded Houston and Portland in place of old Brazilian cities, there was a much larger non-mountainous front. This time, however, I was determined to defeat Siam and convince them that maybe conquering me wasn't their brightest idea ever.
It took some time, as they had a lot of elephants to send our way. Eventually, however, their army was shattered, and two of their cities destroyed, to be replaced by one new American city, Miami. Still, losses had been significant, and by the end we'd lost all frontline troops save a couple Minutemen and a couple Cannons, and had even lost a Great General. Thus we made peace rather than advance on Sukothai, with 37 gpt and Sugar as payment from Siam.
It wasn't time for peace yet, though; Rio was impinging on the expansion of Houston, Portland, and the newly-founded St. Louis, and that meant that Houston had a problem. So after training a few Cavalry, upgrading my Cannons to Artillery (and training a new Artillery), and moving some fresh Minutemen into place, in the early 1940s we invaded Brazil again and took Rio. Apparently Brazil has another city somewhere overseas, as this did not eliminate them from the game. But it eliminated them from our geographic area and meant our northern frontier was now secure.
It also drew the condemnation of most of the rest of the world, who now thinks
we're the warmongers, despite Brazil's provocations and Siam's repeated declarations of war. For Ethiopia (Order) and Austria (Autocracy), the differences are ideological. Thankfully, we still have friends. Carthage is our longest-term friend, though their warmongering against China and Morocco concerns us. The Republic of China is our fellow freedom-loving ally, and our shared ideology has patched over their concerns about our friendship with their enemy Carthage, who has yet to choose an ideology. And Venice still thinks we're pretty cool, and buys more blue jeans per capita than any other country.
I am a bit concerned that Arabia, who had been our longest-term friend, denounced us, as they have many Riflemen not far from our southern border (and yes, Minutemen in the 1940s might be a little bit behind state-of-the-art). The good news is that both they and Carthage are invading Siam, so they have other fish to fry.
The map:
I've also added Universal Suffrage as a tenet of American Freedom. I'm currently attempting to get Freedom enshrined as the World Ideology, and while I may not succeed, China at least applauds the effort.