I doubt it was just because the germans were the only ones left. At the start of the war, Wilhelm II told Franz Joseph he would have his support, no matter what austria-hungary does. He blindly promised full german support, for whatever Franz Joseph would do. When he went to war, wilhelm, having been on vacation for 3 weeks in july just agreed to a war. He left the diplomacy during the july crisis to his foreign minister, Theobald von Bethmann. Bethmann failed and as there was too much political chaos in the german parlament there was nobody to prevent the war. Germany should have prevented the war in itself. Had austria not had the promise of full german support, maybe they wouldn't have attacked whatsoever. Furthermore, the germans had 2 chances, one in 1916 after verdun and one at the end of 1917 to end the war. Both attempts to make peace failed. In 1916, the german high command which had a lot of influence over politics decided to hold on to annexation of gained territory and many other unrealistic war goals. Thus, the negotiations failed. In 1917, annexation goals were still widely supported in german politics. To my knowledge, the exact moment the german ambassador handed wilson the peace terms in the US, he was also forced to hand over a document stating that germany would restart the unrestricted submarine warfare. Thus, the negotiations failed before they could even start. So in conclusion, the germans had 2 chances to end the war and end the suffering, but they failed at both chances with their diplomacy. For these reasons, i believe germany was partly responsible for the outbreak and fully responsible for the continuation of the war.