Before the US Navy eclipsed the Royal Navy one of the closest threats to the Royal Navy's dominance of the high seas was the German U-boats of WWI. The German Naval program started in 1898 by Alfred Von Tirpitz quickly created the second largest navy in the world second only to the Royal Navy. Using one of the most advanced submarine programs the Germans were able to successfully change strategies of luring capital ships toward subs to using submarines against more vulnerable merchant shipping.
When the Germans were starting to break the English's supply chain with submarines the USA placed an ultimatum that if the Germans did not halt their unrestricted submarine program the US would declare war on Germany.
Had the Germans ignored this request they could have starved the British out. With anti-submarine warfare in its infancy - resorting to ramming being most popular among captians.
Why did the Germans wait until it was too late to reinstate their unrestricted submarine warfare in WWI?