Barry Allen is well known for his heroic adventures as The Flash, but at one point in time he served as a soldier during World War 2. While Barry Allen has been around as the Flash for a long time, Barry was introduced long after World War 2 in 1956's Showcase #4.
Jay Garrick, the original Flash, was created during World War 2, and served alongside many other superheroes, such as The Green Lantern, Alan Scott, and Wild Cat. However, Jay eventually retired, and years later Barry would take up the mantle of the Flash, though he never actually served in war time like his predecessor. However, one adventure in Germany would change that, as well as Barry's perspective on war, combat, and death.
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The story begins in Brave and the Bold #28, where Barry Allen meets with scientist Dr. Garton in Belgium. Garton invited the Flash to assist in testing a multi-spectrum laser that can change its own speed and frequency over time. In order to study the exact changes in the laser's frequency, The Flash would run alongside the laser over a period of time and monitor the laser. However, as soon as the experiment begins, and the Flash runs alongside the laser, Barry's own vibrations begin to clash with the lasers own changing frequencies. This causes Barry to get drawn into the beam itself, and eventually get hit by the beam, leading him to blackout. When Barry awakens, he finds himself in Belgium, but during the winter time, a complete change of scenery from the sunny Belgium that he left behind. The Flash also realizes that his leg is broken in two places, and despite a time-rift still existing (giving Barry a means to return home), he cannot run fast enough to return. After exploring his surroundings even further, Barry spots Nazi soldiers in a nearby town, realizing that he was in the middle of World War 2!
Upon being spotted by the Nazis, Barry does his best to outmaneuver them, even creating a snowstorm to put distance between them. However, Barry soon gets caught by a Blackhawk soldier, an elite group of soldiers that served alongside the allies during World War 2. Initially suspicious of Barry, due to his red uniform (and the fact that the "real" Flash is already fighting on the frontlines), other Blackhawks soon emerge and convince their leader that Barry is an enemy of the Nazis. After providing Barry medical treatment for his leg in an abandoned home, the Blackhawks listen in disbelief at Barry's story of time travel, believing him to be crazy. After Barry confirms his story by correctly naming the Manhattan Project (something that was top-secret at the time), the Blackhawks in turn mention how they were looking for a place to assist ground forces. However, the discussion is cut short when Nazi troops find the heroes, with one soldier giving Barry Allen a gun. Barry, opting for no casualties on either side, grabs several dozen bricks nearby and throws them at super-speed, incapacitating the enemy forces. However, while this does end the fighting, the leader Blackhawk is less than happy with Barry Allen's approach to war and combat.
The Blackhawk leader explains to Barry that in wartime, they either kill or be killed, and that there can be very little room for mercy. Barry, reaffirming his vow to not kill while in the Flash uniform, is countered by the rest of the Blackhawks who, by wearing their uniforms, vowed to fight and, if need be, kill as many enemies to stop the war from taking anymore lives. When the heroes leave their hiding place, Barry internally tries to rationalize warfare, deciding that no man truly wants to kill another, but in some circumstances it might be necessary. Barry, not wanting to be a detriment to the Blackhawks, dons an army uniform to cover his Flash uniform, deciding to fight as Barry Allen, soldier, rather than The Flash, superhero. Over the following weeks, Barry gets involved in, and witnesses, a lot of combat, though the story leaves it ambiguous as to whether Barry takes a life or not. Eventually, Barry's leg heals properly and he makes his way back to the time-rift, and after a long discussion about warfare and the future with the Blackhawks' leader, Barry returns to the present. When Barry returns however, he has a renewed sense of appreciation for all soldiers who gave their lives over the years, believing them to be the real heroes of the world.
The debate of superheroes killing their enemies has been discussed both by fans, and by the heroes themselves. For example, Batman refuses to kill violent enemies like Joker on principle, while heroes like Wolverine have a history of murdering villains with very little remorse. This story adds an interesting spin on that idea, as it puts a hero who doesn't kill into a scenario where soldiers, regardless of morality, die in battle. The Flash, in this case, was not facing an enemy like Captain Cold or Reverse Flash, but was fighting in an actual war against enemy soldiers where people died everyday.
Barry's answer to his dilemma as to whether or not he should kill was also interesting, choosing to "put away" The Flash, refusing to go against his superheroic principles, and instead serve in war as Barry Allen. The Flash's reflection at the end was also touching, as superheroes pale in comparison to the real heroes who fight and serve.