I am talking about present day, but since you bring up WWII, France was toast without the US.
Present day example of idealism is the amount of money as a percent of GDP that these countries were contributing to their OWN defense prior to Trump putting the hammer down. They are idealistic and naive enough to believe that climate change is more of a threat to them than Russia. Trump had to warn Merckel not to be so dependent on Russian oil. She didn't listen and he imposed sanctions that slowed down the construction of Nordstream 2 to save Germany from being dependant on Russian oil. Dumb Biden removed the sanctions and what do you know, Russia attacks Ukraine as they no longer had to worry about their oil flowing through Ukraine to Russia. The pipeline was never used, but Germany quickly realized that going "green" so quickly and being fully reliant on Russian oil was a bad combination. In short, they had to learn the hard way.