>>27706347Mecanized warfare realy became a thing in WW1, with the massive use of artillery and machine guns, so I don't think it would change a lot. Technology is basically the same now, just a bit more advanced.
You shoot guns at beasts, beasts die.
Beasts are dumb, they are not trying to cut off your communications and supply lines, they can't reliably sink your cargo boats like a float of submarine could, they can't down your planes. so as long as you have a working economy you should be ok. Heck if anything that could speed up the developement of some technology such as helicopters, which would definitely be a good thing to have in this context.
Now if we're alking pre-WWI with militaries still mainly about big battle lines and heroic cavalry charges, I can see them being a bit more fucked depending of the number of beasts.
Though the "gentlemanly" aspect of war wouldn't eve, be a concern agains't mindless enemies. See how they were machine gunning Zulus well before WW1.