WAR is designed to allow cross-era comparisons. Note it doesn't tell you what a player would have done if he'd played in another era (that's probably impossible to know), it tells you what he did against the competition he actually faced, and then puts that in a common currency that you can use to compare how someone else did against the competition that they actually faced.
The only reason that Trout's WAR scores are so good is that he's really good at baseball.
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