There are a couple of ways to answer this. I’d argue that, in order of formidability, the most determined enemies the United States has faced in battle are:
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Themselves (the combined military forces of the Confederacy and Union).
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The military forces of the Empire of Japan (the Imperial Japanese Army, Navy, and Army Air Forces).
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The military forces of Nazi Germany (the German armed forces, or the Wehrmacht, consisted of the Heer (Army), the Kriegsmarine (Navy) and the Luftwaffe (Air Force).
Some caveats to my interpretation of this question are:
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In my determination of formidability I will look at the American military vs. only a specific foreign military (not taking into account the alliances of either country).
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I will not take into account a foreign military as a whole (for example the Wehrmacht in its entirety), but only the forces that were deployed against the Americans.
A lot of people would rank the Wehrmacht first on this list, and indeed, at an individual level, the Germans deployed during WWII some of the most highly effective and professional soldiers ever to take the field of battle. In many American engagements against the Germans however, we faced an adversary that was overextended (fighting a war on multiple fronts), was heavily conscripted (enlisted soldiers were in many cases not Germans), and was largely non-mechanized (the Germans depended on horses and borrowed civilian trucks and cars to supplement their mechanized forces throughout most of the war).
The Japanese Imperial military on the other hand was an ideologically determined foe (conscripted soldiers would almost always rather fight to the death than surrender), was entrenched (Americans largely fought the Japanese from defensive positions they had spent years building on islands), and was tenacious as all hell.
During the Pacific Campaign in WWII, the Americans took to deploying forces in a ratio of 5:1 in order to ensure the success of their operations. On Iwo Jima alone, 110,000 U.S. Marines, U.S. Soldiers, U.S. Navy corpsmen were required to displace approximately 20,000 defending Japanese soldiers.
However, given the criteria I outlined above, by far the most formidable adversary the United States has ever faced, is ourselves.
The American Civil War resulted in the combat deaths of approximately 214,938 Americans (broken down to around 140,414 Union and around 90,000 Confederate) at a time when the population of the United States as a whole was 31 million. WWII in comparison resulted in the combat deaths of 291,557 Americans at a time when the United States population was 131 million (or 4.3x larger). Those statistics are staggering in their own right.
The American Civil War, from an ideological perspective, was a fight for the existence of the United States itself. When taken into account that the American military was then, and remains now a military that derives the majority of its enlisted and officer corps from southern states and when also taking into account that some of the greatest military minds of that generation, chiefly among them Robert E. Lee, declared their allegiance to the Confederacy rather than the Union, the American Civil War represents the closest the United States has ever come to complete dissolution in the face of a determined adversary.
– Patrick Dugan
Theodore Lee is the editor of Caveman Circus. He strives for self-improvement in all areas of his life, except his candy consumption, where he remains a champion gummy worm enthusiast. When not writing about mindfulness or living in integrity, you can find him hiding giant bags of sour patch kids under the bed.