In the late-nineteenth century, was the United States essentially isolationist, essentially expansionist, or a combination of both?
In the late 1800s, the United States was definitely expansionist. This was a time when the US was being very aggressive about getting involved in other parts of the world.
If we stretch the idea of the "late nineteenth century" we can look at American expansionism as far back as the "opening" of Japan in the 1850s. But that is not really late in the century. So we should look instead at the US expansion into Hawaii and Samoa. We should look at the Spanish-American War, which was caused by a US desire to prevent Spain from abusing its colony in Cuba (as well as by a strong desire for empire). We should look at the fact that we then took the Philippines as our own territory even in the face of a strong independence movement that required a relatively long war to defeat. Finally, we should look at the Open Door Notes that the US was sending to demand more of a part in the exploitation of China. All of these things show a strong strain of expansionism and no real hints of isolationism.