I explained more here, but leftists oppose NATO because NATO is the millitary alliance of the world’s imperialist powers. This group of countries uses this alliance to prevent the global south from going against it and liberating themselved from foreign plunder via overwhelming financial domination. The way imperialism tends to work in the modern day is countries like the US, France, Germany, UK, etc expropriate vast wealth from countries in the global south, similar to how capitalists steal value created by the working class.