What if the technology for a useable, reasonably compact radar was around in WWI?
Specific applications I could see would be proximity fuses for artillery shells (which makes the trench gridlock even bloodier) and of course fire control for ships. Radar for ships would make fighting in the cloudy North Sea a somewhat easier affair and would probably allow for the Royal Navy to root out the High Seas fleet more easily (unless the British happen not to embrace the new technology).
Airplanes of that era are too light to carry radar sets, but radar equipped zeppelins could be used as primitive AWACS to discourage casual reconnaissance and provide early warning.