What are some of the biggest effects of technological developments in 20th century wars?

Most are considered to be "Force Multipliers". That is they increase the

military capability simply because of their existence. Consider for example

the use of smart bombs. WW2 aircraft were often required to make many

repeat attacks to take out a target. Thousands of bombs were expended

on open fields miles away from their intended target. Today one aircraft

with a single crewman can place a bomb into a designated window. The

cost of such technology is high but the savings in risk to aircrews makes

such expenditures a good trade off.
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