I've been reading To End All Wars recently and it makes it seem like England, France, and Germany were right on the cusp of class war before August of 1914. I'm curious as to how accurate that is. How prevalent was socialist sentiment in that era? Were tensions between capitalists and progressives nearing a flashpoint or was it comparable to modern day socialism where the politics are more or less passive?
Not as close as you might think. Although socialist ideology was quite popular throughout Europe in the late 1800s and early 1900s, the early 20th century saw an enormous surge in nationalism, fueled by the rise of popular liberalism after the Enlightenment and the French Revolution. This was all part of the gradual trend away from monarchial and dynastic rule towards the modern-day Western nation-state. For most countries at the time, growing national pride overshadowed many of the class-fueled tensions of the time - some historians consider this one of the most prominent obstacles to the Pan-European socialist revolution that Marx had predicted.
That being said, it was nothing like it is today. Marx's vision was still alive and well in the minds of many prominent socialist thinkers and leaders, and there were still many autonomous parties and organizations promoting revolutionary socialism throughout Europe. The point is just that in the years prior to WWI, this was largely overshadowed by liberal nationalism and of course militarism.