Rigid steel armor and battle axes were a huge tactical advantage at the battle of Visby (1361)(defenders had mail armor, iron helmets, boiled leather arm & leg armor).
The big advance was stirrups. They allowed a rider to brace himself against the horse and swing a heavy blow without knocking himself off his own horse. It also made it much easier for a rider to control his horse with his legs and therefore be able to use both hands for fighting.
The weapon itself may not really qualify because longbows were used throughout Europe, but the English developed the effective use of longbows in battle, and it gave them quite an advantage for a while.
I think cannon is the biggest one. Before it military strategy was heavily based around seige warfare and castles. Anyone rich enough to throw up some walls could be a minor military power. After cannons, walls and castles became close to useless, and both warfare and society fundamentally changed.
In the 1934–1939 period, eight nations developed, independently and in great secrecy, systems of this type: the United States, Great Britain, Germany, the USSR, Japan, the Netherlands, France, and Italy. In addition, Great Britain had shared their basic information with four Commonwealth countries: Australia, Canada, New Zealand, and South Africa, and these countries had also developed indigenous radar systems. During the war, Hungary was added to this list.
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So, no, the US Navy didn’t have anything that everyone else didn’t also have.
Radio and other form of long range communications.
The aircraft carrier changed several centuries of naval warfare by big gun warships.
Canned food enabled armies to become much larger
Bearing in mind we’re looking at things from the tactical and strategic angle rather than the technology angle:
Fighting in formations rather than as individuals. From fighting in a phalanx to Roman legions to British troops at Waterloo forming squares against cavalry.
Improvements in communications - telegraphy then telephone then radio - allowing vastly more rapid communications over ever-greater distances.
The aeroplane.
Logistics. Napoleon cocked it up; Wellington was the master of it.
I think drachillix is referencing that the US implemented it faster and better than the Japanese and so gained a tactical advantage in both detection and firing solutions for naval ships that were otherwise comparable.
According the OP’s guidelines, it seems appropriate for this thread.
cite: The book Last Stand of the Tin Can Sailors, published in 2005, about the battle of Samar. Read it two months ago and it stated the use of radar for firing solutions was a definite boon in that the US ships had it and the Japanese ships didn’t.
EDIT: That may not be what drachillix was referencing specifically since I can’t read his/her mind, but I think it’s still on-topic.
Not just radar, but particular wavelengths of radar. Many nations had radar good enough for navigation, but accurately detecting ships and aircraft takes some precision. US radar got sensitive enough to detect submarine snorkels/periscopes.
For centuries, Venice’s position in a very shallow, difficult to navigate lagoon meant it was immune from invasion, army or navy. It was a major regional power. It did not even need city walls. then Napoleon marched up to the shore with cannons able to reach 3 miles to the city. The city fathers surrendered without a fight.
Similarly, the small city-states of Italy and to some extent germany disappeared when military tech got to the point where a small city did not have the infrastructure to equip an army, and could not field one big enough to defend against the massive armies that the greater european powers could put in the field.