The main reason why the United States largely shied away from imperialism prior to 1890 was because it was still a relatively "fragile" nation with a host of its own issues to work out--such as the slavery issue that eventually led to the devastating Civil War.
Trench Warfare was a form of military conflict in which opposing sides fought one another from trenches facing one another. WW1 warfare introduced the first large-scale use of chemical weapons as well as bringing in airplanes and tanks for the first time.
