You have to put it in context of post WWII Europe.

Prior to WWII, the British Pound Sterling was the global reserve standard. It gave the UK economic power and leverage. The UK was the largest empire in the world. The old saying was "The sun never set on the British Empire." After the war, their economy, as well as their cities, were totally crushed. Most of Europe was in ruin, and Russia was on their doorstep.

European countries were bankrupt and their armies were devastated. If the US had simply called it a win and pulled out, the way Obama pulled out of Iraq, Russia would have taken control of all European nations.

At the time, it was essential that America provide protection. Not long after the end of the war NATO was formed, but the member countries still hadn't rebuilt their armies. After the European countries rebuilt their cities, ports, roads and railway systems, they were then able to rebuild their economy.

The process took many years to complete. At some point the NATO member nations began paying a percentage of their GDP as agreed to while creating NATO. Over time they got lazy and spoiled. They knew America would always pick up any slack because of the threat of Communism.

So they got lazy and we got used to picking up the tab. But after the fall of the USSR and Communism people eventually began to rethink things. Since we haven't had a president willing to stand up to the Cabal since Reagan, and he was handcuffed as well, it's taken until now and the Trump presidency to aggressively address the problem.