Well, since I helped start his mess...
It's a real good question. Much depends on how fast it develops and under what circumstances.
Diplomatically, hard to say. Wired communications already exist between world leaders and generals, so I don't imagine anything altering the onset of WWI or mobilization schedules much.
A big question is how far does broadcast radio advance? How quickly will it economically diseminate into your average household? OTL you had radio in most households in the 30s or so, so twenty years' from the POD you might have broadcast news reports of Franz Ferdinand, or whatever ATL spark hits (assuming relevant infastructure and supporting tech are there). If the people have instant acess to such info I could see this sparking outrage earlier. Plus I doubt the states of Europe are going to allow uncensored broadcasts at this point. Could the horrific and pointless death tolls for negledgable gain be known to the public more clearly, even if via "pirate" radio?
With radio communications rather than wired telegraphy you're a lot safer from losing communications with a random shell hit, but conversely unless you have a way to encode your transmissions you're broadcasting your plans to anyone with a receiver.
Of course for unit and vehicle communications a radio transmitter could have major effect. Even if a transmitter is still too big receivers can be really small allowing orders to get sent out directly. A spy or reconnaisance troop with a small enough transmitter could be far more effective in delivering information sooner.
Planes will still be without radio for a while, except maybe simple passive receivers to get orders or direction.
For naval warfare, this is revolutionary, as now ships don't need visual line-of-sight to flash orders. Ships and subs can communicate with shore from over-the-horizon. And transmitter size is far less of an object.
And another big big possible butterfly is Radar. The reflection of radio waves was known since the 1880's OTL, and Tesla OTL came up with an idea for primitive radar in 1914. You could logically boost radar in time for appearance/fielding by WWI, which is, again, a monumental effect on navy affairs. Ships capable of engaging beyond line of sight, unhampered by smoke or fog...major butterflies possible, particularly if one side adopts it and another doesn't (see US vs. Japan WWII for OTL example there).
Radar will help in detecting air raids, though we'll be talking really primitive radar and smaller cross-sections (reduced detection range) with wood-and-fabric planes.