Fairly accurate. By 1865, if you count all Union forces, the north could field over half a million men at arms. Remember, though, the British had an arguably stronger navy at the time (so unless the U.S. marched rather than sailed to their next conflict, those troops wouldn't have been worth much in an offensive international conflict). Still, it can be fairly stated that the U.S. in 1865 had the largest professional (big distinction there) army in the world. If you consider the fact that the U.S. had a converted wartime economy and industrial infrastructure, not to mention hundreds of coastal and inland forts (all of them well-staffed), you could make a case for the fact that the U.S. was probably the most un-conquerable country in the world at that particular moment in time.
[Sources: Personal Memoirs of U.S. Grant; James McPherson, 'Battle Cry of Freedom']
Here are some previous answers that may help.
https://www.reddit.com/r/AskHistorians/comments/yvfvh/what_did_europeans_think_of_the_military/