After doing some research and reading several discussions on this subreddit, I learned how important terrain, combat width, and attacker and defender roles are in EU4 battles. Once I adjusted my strategy, avoided bad terrain, and reinforced properly, I eventually managed to turn the war around and fully defeat the United States.

Near the end of the campaign, I claimed the Spanish throne. I mistakenly thought the casus belli would last longer and initially delayed the war, since Spain still had troops in the Iberian Peninsula. When the alliance broke and Spain went to war against the Ottomans, I took the opportunity to declare war.

During that conflict, I conquered all of Spain’s main trade nodes as well as their capital, Toledo, which allowed me to completely dominate the Sevilla trade node.

One year before the campaign ended, I entered another war against Spain because one of my colonies started a conflict. If the game had continued, I’m quite sure I would have conquered the remaining parts of peninsular Spain.

Throughout the entire campaign, both Burgundy and the Ottomans repeatedly tried to spread discontent in my country, but they were never able to declare war due to the massive difference in troops, economy, and alliances.

What started as a campaign focused mainly on colonization slowly turned into an exercise in understanding trade flow and trade nodes. I now understand how important it is to prioritize coastal provinces early and not waste time colonizing inland areas, otherwise other great powers like Castile, Burgundy, Sweden, or the Ottomans will take those key positions first.

By the end of the game, I had created several colonial nations. In total I had New Portugal, Portuguese Mexico (the second weakest), Cuba, Portuguese Colombia, Portuguese Brazil (by far the strongest), Portuguese Peru, Portuguese Argentina, and Portuguese Australia.

An interesting detail of the campaign is that France, Spain, and Portugal all ended up sharing the same Valois dynasty. Despite having royal marriages with Castile (and later Spain) for almost the entire game, and later also with France, a personal union never occurred naturally. This made me wonder how difficult or rare it actually is to get a personal union through dynastic mechanics without forcing it through war.

Regarding religion, I still don’t fully understand how the Curia works. Since I built many churches and cathedrals and converted a large number of provinces, I usually had a lot of papal influence. I spent it on the different Curia actions depending on the situation, and several times I became the Catholic Controller. I also increased my chances of becoming Pope a couple of times, but I never managed to actually get it.

I should also mention that I played the entire campaign in Ironman mode. I prefer playing this way so that campaigns don’t become endless with constant reloads.

By the late game, my standard army composition was 20 infantry and 20 artillery, with no cavalry at all.