Which military product design innovation of the early 20th century was initially rejected by military leadership but later became essential to modern warfare?

Throughout history, military product design has dramatically transformed battlefield capabilities and tactics. From ancient weapons to modern technology, these innovations have shaped the course of warfare and often transitioned into civilian applications as well. This poll tests your knowledge about a groundbreaking military product design that revolutionized combat strategies and influenced subsequent military developments.

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The tank, which British military leaders initially dismissed as a 'mechanical toy' with limited battlefield value
The machine gun, which generals considered wasteful of ammunition and unsuitable for 'gentlemanly warfare'
The military aircraft, which was viewed merely as a reconnaissance tool with no combat potential
The submarine, which admirals declared 'underhand, unfair, and damned un-English' in naval combat