Radar is the decisive 1935 development for World War I

Which single development from 1935 should be taken back to World War I?

Radar, because it improves detection, targeting, air defense, naval combat, and artillery across the entire war.

Radar is the immediate choice for its applications across the armed forces. Naval uses include gun targeting and support for the Dover barrage, with Jellicoe singled out as a commander who would benefit. On land, radar is proposed for artillery counter-battery work. It could also help direct fighters against aircraft and Zeppelins. Other answers favor preserving manpower with antibiotics or supplying a better engine, so radar's priority reflects the emphasis on immediate combat effectiveness.