Well, it is true that the UK would have lost and been comprehensively defeated if the US did not come in on their side. They couldn't even feed themselves without US cargo ships coming in, and without the 'lend-lease' destroyers and aircraft, there would have been no way to protect the convoys either, and that would have been the end of the UK as far as Adolf was concerned, whether they officially 'surrendered' or not. This is what I meant by 'neutralisation,' and if the US didn't make all their efforts on the behalf of the UK, even well before the US got officially into the war, I don't think you would have heard Churchill making quite so many speeches about 'fighting them on the beaches' as he did....... And certainly, for all intents and purposes, the UK was comprehensively kicked out of the Far East and SE Asia in quite short order......
Same is true for the US vs Japan. If Japan could knock the US out of the Pacific, then they would have achieved their objective, and it would have been extremely difficult for the US to have come back in, especially with the war in Europe still raging. As for submarines, the Japanese had just about the best submarines in the world at the begining of the war, and one point in particular made them imminently suitable for West Coast patrolling, and that was the incredible range these boats could go without refueling or resupply. It is important to remember that just before Pearl Harbor, there was something like a dozen IJN subs quietly patrolling around the Hawaian islands, and they had been there for almost a month! Japanese fleet submarines were specifically designed to operate off the US West Coast for extended periods (and in fact did so on a few occasions!), and the only reason they did not in fact do so as a primary mission was because of the Japanese Admiralty, not because there was any limitations on the boats (and a lot of them carried their own airplanes too!). Think in terms of the U-boats off the East Coast, and the damage they caused. They never numbered more than about a dozen boats at any given time 'on station,' and usually a lot less. When you figure that the Japanese had the largest fleet of submarines in the world in 1941, then the idea of fairly constant submarine patrols on the West Coast becomes a very grim reality, and even worse if it is backed up by a periodic surface task force coming out of a captured Pearl Harbor....
But the facts are, the Japanese appear to have gone completely stupid as far as the submarines were concerned, and never adapted their tactics for anything besides fleet actions. Not because they couldn't, they just didn't! The US Navy however, learned a LOT from the Germans in WW1, and scattered their subs across the Pacific to seek out and destroy merchantmen in coordinated attacks, and if a warship happened along, they would sink that too, but primarily attacking the merchant fleet. And the Japanese casualties from this campaign were staggering! It got so bad, that by the tail end of 1942, the Japanese were already forced to use submarines to bring in supplies to Guadalcanal, submarines that could have been sockin' it to the US merchant fleet instead.
It really is a bizarre scenario with no rational explanation, as the initial Japanese sub designs after WW1 were the result of German engineering, with a whole bunch of German engineers and former German U-boat officers specifically brought to Japan to teach them about submarines for almost ten years. Apparently, the class on strategy, tactics and objectives never made it into the syllabus!