Social Darwinism was a misreading of Decent of Man to support racist beliefs and ideologies. “Fitness” is determined by the ecological niche for a human population. White skinned Germans would be at a substantial disadvantage living in sub-Saharan Africa without the survival skills and adaptations of the locals in Namibia. There is no universal form of “fitness.”
In a state of nature without laws or moral constraints those with a “strong will” domininate those with a “weak will.” This was accepted even in advanced societies like Greece and Rome that considered slavery to be moral.
Modern civil society evolved to constrain those with a strong will through laws, customs, and moral traditions to maintain a semblance of peace rather than living in the perpetual “parabellum,” of all against all.
What ultimately stopped WWI was the stronger will of Great Britain, France, and the USA to defeat the weaker will of the Second Reich military cult and restore a state of peace.
This holds true even today with test of will between Radical Islam and the Enlightened Western democracies. Islam is banking on the West preferring to live on their knees as slaves rather than standing and fighting (once again) for individual freedom.
What ultimately stopped WWI was the entry of the US.
By 1918, all the other belligerents were war-weary and depleted of manpower. Most were also depleted of industrial capacity. Ludendorff's Spring Offensive was an act of desperation to drive the Allies to the bargaining table before the Americans could arrive in force. It failed on all counts.
By summer of 1918, Americans began arriving at the rate of 10,000 a day, and the Germans never managed to sink even a single American troop ship, so it was obvious they were never going to stop coming. After the German failure at 2nd Marne, Ludendorf admitted Americans were arriving faster that he could kill them, so the Germans had better negotiate and end to the war as fast as possible because the conditions Germany might receive were only going wo worsen as their military position continued to deteriorate.