If you were a soldier in "In Another Country" by Ernest Hemingway, do you think you could live more positively?
One of the realities about World Wars I and II was that they proved that entire civilizations and their people could be devasted at one blow. In earlier epochs, the machinery of warfare and the ambitions of war operated on a different scale. Certainly there were conquerors who overtook the "whole" world as shown by the vast Roman Empire and the later vast British Empire, but, as historians agree and assert, the progress and weapons of conquest and the level of devastation in WW I and II were on a different scale.
Part of Hemingway's conscious (or perhaps subconscious) intent is to lament that scale of devastation and the presence of its reality at the personal level as well as at the physical, measurable level as seen in the land, the graves, and the hospitals. Therefore, it is difficult to suppose or suggest that a 21st century person, transported to either World War I or World War II could fare any better than those who were the original occupants of the epoch: the realities of the events and circumstances would be the same for any participant.
In fact there are those who would contend that because so many moral values and ethical positions are lost or nullified in 21st century Western civilization, transported individuals would fare less well because they have not got years of optimism and devotion to the ideals of honor and duty to sustain them through the horrors and large scale devastation (personal and planetary) of either of those wars.