Hmmm.
1. Have the UK/France not guarantee Polish independence.
2. Have war against France start in 1940 and go pretty much as historically.
3. Attempt to evacuate from French coast goes horribly; 35,000 evacuated, scores of RAF fighters lost and a dozen destroyers. 200,000 UK and 100,000 French troops captured.
3. Britain agrees to armistice to deal with other matters; rumblings in India, Italian aggression in East Africa, Turks [Mosul and Cyprus], Spain [Gibraltar], Japan et cetera.
4. Turks join Axis. French fleet surrenders to Germany.
5. Stalin dies under mysterious circumstances. USSR experiences "interesting times".
6. Germans invade USSR in 1941. No Lend-Lease, no Western front or bomber war. The Japanese and (to a far lesser degree) the Turks join in. Things go badly for Soviets but it's a big country.
7. Britain steps up rearmament ready for WW2.5, quiet discussions with USA.
8. Eastern Front becomes bloody quagmire (no change there) but Germans advance far further, capturing Moscow et cetera.
9. Desperate Soviets deploy biological weapons, Tularemia, Q-Fever, Typhus, Plague. Hitler authorises retaliation with chemical agents. Many observers surprised by the effectiveness of Tabun (other nerve agents aren't available in quantity). Soviet transport hubs are drenched in mustards.
10. Widespread condemnation of CBW, "they did it first". Frantic UK and US research into organophosphates.
11. Joint US/UK nuclear weapons project starts but delayed compared to OTL and less resourced (US not at war). Much interest in anthrax and other CBW agents diverts resources. US begins military expansion but on a smaller scale than OTL.
12. It's now mid-1943 and chunks of Eastern Europe are decidedly nasty places to visit. Japan demands that the UK and US stop supplying arms to Nationalist China. When the UK doesn't comply they launch small-scale raids into Burma to destroy supplies and demand the right to inspect cargoes in Hong Kong. UK deployed reinforcements to the Pacific.
13. US issues warning to Japan, implements trade embargo. While Japan has access to oil from the DEI (a German "protectorate") the fighting in the USSR and China is a drain.
14. Anglo-Japanese naval battle in Pacific. British, American, Japanese declarations of war (well the US doesn't declare war on the UK). Germany joins in. WW2.5 is on.
15. US massively expands it's armed forces, but this will take time. More funds to Manhattan Project. Germany establishes bases in Canary Islands, in Iceland and Greenland. US begins Lend-Lease supply on a large scale.
16. German expansion into USSR slows as RAF raids begin. Luftwaffe responds. There is an unofficial agreement not to use CBW agents.
17. Fighting in Pacific and in peripheral theatres. Allied landings in North Africa. Naval battles in Atlantic as the Suez Canal route is effectively closed and UK food imports switch. US forces land in Greenland, with heavy casualties amongst the green troops. Iceland next. Carrier raids on Canaries.
18. It's now early 1945. The atomic bomb is three years away at best, mass produced anthrax is closer. The Japanese are being slowly pushed back (Midway analogue was a disaster for the US). Germans realise their codes are being broken, encryption greatly improved. Ultra negated. The Germans are almost at the Urals but their loses are not sustainable and their minor allies are balking at the cost.
19. The US want a cross-channel invasion in 1946 but they are persuaded by the disastrous invasion of Iceland that this isn't possible. Germans reinforce Atlantic wall defenses. Rocket bombs begin falling on UK, without chemical warheads. Yet. Much fighting in North Africa, naval clashes in Med.
20. Large US build up in the UK for the invasion of Europe. Also large losses from submarines in North Atlantic. USAAF begins mass bombing of German cities, taking huge casualties. Germans respond with submarine launched rocket bombs against US East coast. FDR dies (heart attack), Truman succeeds to presidency.
21. Allies advance, slowly, in the Pacific. Japan uses biological weapons against Hawaii. US forces employ chemical agents to soften island defenses.
22. 1947 dawns. The invasion of Europe is planned for June (weather) but the Allies don't have air supremacy (better German production, less need for aircraft in East et cetera). Fears of chemical agents being employed against invasion beaches. Manhattan project experiences new problem; Xenon poisoning of pile.
23. By April the Allies realise that the invasion will have to be postponed or face disaster. Given the success in North Africa there are suggestions of a landing in Southern France. The Germans don't know this and maintain heavy forces in the west, reducing their capability in the east. Hitler assassinated (look it had to happen sometime).
24. Allies refuse German peace feelers, slowly grind down Luftwaffe. Overlord in 1948, with both sides using poison gas. Slow advance across Europe.
25. 1949 dawns. Atomic bomb tested (it works) in June. Series production ordered to build up a stockpile. By November sixteen weapons are ready along with the aircraft to deliver them. Blitz against the nearer German cites and operational centres with a dozen bombs. Surrender demand. German retaliation with rockets carrying two-tonne Soman warheads (including against US coast). Further nuclear bombs used. Regime change and the formal German surrender in early 1940. More than a hundred million are dead (and that doesn't include the camps). Japan next.