Is the world polarized used correctly:
The issue of imperialism polarized the Americans during the late 19th and early 20th centuries over different beliefs...
If the majority of people were at either extreme in terms of their opinions, then the term, "polarized," is correct.
However, this is relatively unlikely. People with extreme views (whatever they are) are much more likely to express their views, while those who are neutral or undecided tend to remain silent. This can give the appearance of polarization of the population when it does not really exist.