Snipers in the Civil War were a major factor, as their primary targets were officers. Militaries have long understood the value of decapitation of the enemy's leadership.

The French conquered England in the battle of Hastings when they managed to kill the king.

There were so many assassination attempts against Hitler that he spent the last couple years of the war in a bunker, and the war ended the day he died.

True. But in Vietnam, Afghanistan (both Soviet and US conflict), and other environments, targeted assassination of civilian targets became a much bigger thing.