The US became a superpower when it put 3,000,000 men in the field for the Civil War.
It expanded that status with the US Naval victories in the Spanish-American war.
When Pershing won the Battle of Saint-Mihiel on the strength of the US First Army (500,000 men) in 1918, there was no looking back.
In total the US put 2,000,000 combatants in France by 1918. Nobody had ever put that large an Army that far from home before...unless they walked.
You write well...
The most amazing power was Japan.
They went from a backward feudal state in the 1850s to a world power by 1905.
Agreed. I think the Civil War transformed America to the level of world power. It may not have felt like it at the time but that is the war I would say changed everything. The subsequent wars were the manifestation of the changes the Civil War had wrought.