About a theoretical SPD-lead government in 1914 Germany: They'd have to deal with the mighty conservative powers (big agrarians of the East, capitalists, military, Prussian parliament - Prussia was essentially 2/3 of the empire, and other than the German parliament (which had first-past-the-post), Prussian parliament was elected by an antiquated system: All citizens in every constituency were divided into three classes, the rich, the middle, and the poor, so each class paid one third of the taxes. Of course, this greatly favored the Conservatives. [In one Prussian election, they got more than 100 seats compared to 7 {yes, seven!} for the SPD, although the SPD got more than one and a half the votes of the Conservatives.]).
However, they might do some smarter things: They wouldn't start unrestricted submarine warfare, they'd be willing to make a compromise peace when it becomes clear that the war will drag on and take many victims, they wouldn't force hundreds of thousands of Belgians to work in the German industry...