How about this?
POD July 1940.
The French Fleet at Mers El Kebir gets the 'Interned by the US' option - and takes it.
The French Fleet transits Gibraltar, headed for Martinque. The RN escorts it as far as Cadiz, and then the USN takes over, easy as pie.
Hitler, already frustrated by Churchill, and fealing his oats after France, flies into a rage. Not realizing that the escrorting warships are now USN, and not RN, Nazi u-boats attack the fleet - somewhat ineffectively. However they do sink a US DD and a US CL with loss of life before withdrawing.
No, this does not get the US into the war in July 1940.
It does infuriate the US public. Roosevelt is able to get Lend-lease passed much earlier and it is more generous. Additionally, the US exclusion zone in the Western Atlantic is set up earlier. The RN and USN start cooperating much earlier than in OTL, especially on ASW and convoy tactics and technology.
All of this leads to an escalating series of incidents in the Atlantic between the Kriegsmarine and USN. The 'undeclared war' is in full swing by October of 1940, and US casualties mount.
Butterflies lead to O'Connor not being captured, and Rommel is sent earlier to Tripoli. Additional butterflies lead to Crete holding in 1941.
The kicker comes in April of 1941. The Bismarck, rushed into the Atlantic early, has its fateful encounter with HMS Hood, and, as in OTL, blows HMS Hood away. The German Battleship is not damaged, and Bismarck and Eugan pursue HMS Cornwall, a CA that had accompanied the Hood (with the earlier sortee, HMS Prince of Wales is not ready). Bismarck catches Cornwall after the latter meets up with what Bismarck thinks is the HMS Warspite. Bismarck engages both, not realizing that the other ship is actually the old US BB New York. The New York is outmatched by the modern Bismarck, and sunk, but not before damaging the Bismarck's steering, enabling the rest of RN to catch it and sink much as in OTL.
The New York is the kicker. The US DoWs Germany in May of 1941. Hitler, facing a real war in the west, cancels Barbarossa, and engages more heavily in North Africa, where the port limitations keep him from expanding too much.
Japan DoWs per OTL.
With no Soviet in the war, Germany is far stronger. However, ALL the US aid goes to Britain. No starvation, no real convoy issues (no Murmansk run), and an earlier jump on ASW technology mean that by late 1942, the curve is in the Allied favor. The German Army is huge, but a series of brutal back-and-forth battles in North Africa give the Allies the battle experience and doctrinal expertise to take the Germans on in a theatre the Germans can not easily reinforce.
Italy is knocked out by early 1944, and the Allies do Overlord in 1944, at the Pas de Calais. The Germans almost throw them out, but overwhelming Allied logistics save the day. Brutal battles in France, coupled with a Soviet May Day 1945 stab-in-the-back in Poland mean the US Army takes Berlin in early 1946. The Germans actually drive the Soviets back as far as Minsk before the Soviets recover.
Japan goes as in OTL.
So, in 1946, the British are tired, but not exhausted. They have actually taken fewer casaulties, and been even tighter with the United States. With no Soviets, the Germans had tried games in the Middle East, but the British Empire proved it's worth.
With a hostile USSR (no war-time cooperation, no rose-colored glasses for Roosevelt), the US bolsters the British even more than in OTL. Britain is much quicker back on their feet, and with the Soviets detonating a bomb in 1949, the US feels it needs a strong ally. Massive US investment in Europe provides a road for British firms to compete. The Marshall Plan creates a US market in Europe, yes, but also a British market. The German rocket scientists were divided between Britain and the US, not the USSR and US. British Aerospace technology is given a boost as a result.
The Empire still goes, but as the truth about German death camps, and the tacit Soviet alliance with the Nazis comes out, Communism has much less appeal. China is divided, with the North Communist and the South Nationalist, and British Hong Kong becomes the door to that investment 40 years earlier than in OTL. The CW is stronger, especially in Asia, and British firms reap the profits of sound investment.
How's this?
Mike Turcotte