Better still, eliminate Friedman. Once he proves the rotor machine can be solved, in 1920, it's just a matter of time. If it takes until, say, '39 to even know it's possible, Enigma may get broken, but not in time to help. I'm also going to presume the Type 97 machine (
PURPLE) doesn't get broken in time by
OP-20G, either; offhand, I can't say if JN-25 would get broken anyhow. J-19 probably does. And without the security paranoia in the U.S. over
PURPLE, it's possible the
stupid decision to seize & copy codebooks from
Nisshin Maru II ('40?) doesn't happen.
Presuming these things, things look a fair deal bleaker for the Allies. Less interdiction from Malta of supplies to DAK. Less (no?) chance of discovering the Germans are reading Black & using the information Fellers provides to counter Brit plans. Somewhat less effective ASW in ATO. Less warning at Coral Sea & Midway (Hypo still reading callsigns, I presume, so some hints); likely defeats at both. (Halsey KIA Coral Sea? Fletcher too, or at Midway?) Perhaps no Doolittle stunt, tho; maybe this, &/or loss at Coral Sea, butterfiles Midway. No warning of von Kluge's attack after D-Day, so no Falaise Pocket.
On the upside, U.S. reading (& actually paying attention to) J-19 means IJN gets a mightily hot reception 7/12/41...


With a need to pay better attention to other intel probably means the '44 German attack in the Ardennes is anticipated & destroyed. Maybe (but probably not

) better use of DF & ASW a/c in ATO (net better than OTL?). More effective USN subs in PTO from war's start (the
maru code made a huge difference when it was broken again 1/43 OTL), so maybe Nimtiz doesn't pull boats off the firing line for supply missions or close recon of Japanese ports, instead targeting tankers & DDs, while using mining & traffic analysis to bottle IJN ships, which ends the war around 2/44...

Presuming Halsey KIA, Spruance crushes IJN at Leyte Gulf, the largest single sea victory in history (& likely to remain so); unfortunately, G.H.W.Bush drowns after his TBF is shot down before he's rescued...

Spruance runs for Pres in '52?

(Defeats Ike?

)