Historically, Germany was supposed to send 80-90% of forces against France, Austria was to hold back Russia and Serbia, and Britain was not even supposed to enter the war.
Pre- World War 1 was a diplomatic disaster for the Kaiser, with only A/H as a stable ally. However, the Schlieffen Plan could still work, though it would likely need Austria/Hungary completely defending Germany’s eastern rear.
In extremely-simplified, “experimental” games I have played with just France and Germany, Germany wins only leaving Prussia’s armies behind. When Britain is added, Germany must send 100% of starting forces and the first 2 or 3 turns (5 inf, 5 art each turn) West to capture Paris. It takes another turn to secure any gains, all for a total of 6-10 turns.
(This is playing 1-space moves.)
Overall, it is difficult, but possible. Also, this was “experimental”, leaving out details like Turkey taking India from UK, and Austria losing to Italy.