A few points...
The reaction of the Empire to Dunkirk and its aftermath - in particular the percieved danger to Britain - was quite astounding. Countries like (for example) Canada, whod been supporting the war but being careful not to ruin their economy in the process (a quite reasonable caveat), changed pretty much overnight to an attitude of whatever we can we will do. With no caveats.
I think we're talking past each other on this one. Yes, historically the empire did pull together after Dunkirk to an extraordinary degree. The question is whether that outpouring of support would have still happened if Britain looked like a sure loser in the aftermath of Dunkirk. Would it have happened even if it was not in the best interests of the Commonwealth governments for it to happen?
Historically, supporting Britain was rational for the Commonwealth countries. Canada to some degree, and especially Australia would have been threatened if Britain had fallen. However, supporting a common empire-wide effort only made sense from the point of view of the commonwealth countries if that effort had a reasonable chance of succeeding. If the commonwealth governments felt that Britain would inevitably fall, then the rational course for those governments would have been to build up local defenses rather than waste limited manpower and armaments in a lost cause.
Are you claiming that support would have come even if giving it was irrational or perceived as irrational? If so, we'll have to agree to disagree on that one. Government and publics aren't always rational, but I'm not going to buy an argument that claims certainty that a country would react against their perceived best interest.
Minor countries who fancy taking a few bits off the Empire while Britain is preoccupied face a number of issues.
The Empire still had considerable local forces based all around the Empire.
Second is the sheer risk of tweaking the lions tail. Sure, Britain looks on the ropes. But if they arent.... it isnt worth the risk. Not considering the history of the Empire it isnt... Now if Germany invade or conquer, thats a different matter altogether.
Here, I think we're saying about the same thing in different ways. There are two issues: (1) the local balance of power between the empire and those who would have been tempted to take a chunk out of it. If you look at that balance of power in June/July of 1940, you'll see that it's pretty heavily weighed against the Brits on quite a number of potential battlefields, especially in the Far East. The British certainly wouldn't have been able to take on Japan more effectively in summer 1940 than they did in late 1941. (2) Perceptions of British power. In the years between 1783 and 1940, it was rarely a good idea to be on the other side of a war from Britain. Historically that track record kept most of the potentially hostile powers from attacking Britain at its weakest. At what point would the perception of British power crumble? As I stated, probably not at the point this scenario ends up at.
Historically, the fall of Singapore dealt British prestige in the Far East a blow it never recovered from. If Japan had entered the war in summer 1940 and done as well as it did in the it might have triggered a scramble to grab cuts of the British empire.
Yes, the attitude of the USA to Japan was pretty much one of undeclared war - their actions up till PH show this, these wern't the actions of a country prepared to be conciliatory.
Here, I think it depends on time-frame and definition of 'undeclared war". I'm talking about the period right after the fall of France and to some extent continuing until the Japanese occupation of southern IndoChina. I would be interested in hearing about US actions during that period that could be considered acts of war, declared or not. The US gave some military and economic aid to the Nationalist Chinese. Other than that, we really didn't do much. After the Japanese took the rest of Indochina (a year after the period in question) the US did take much more aggressive actions, including some that were arguably acts of war.
There is also of course the reaction of the USA to a less effective Dunkirk. Probably not too much initially, they were still looking at France. Once France feel, and the French fleet looked in danger of falling into German hands, Congress basically had a collective fit at the idea, passing an enormous peactime navy bill, basically giving the navy a blank check of any amount.
Now of they did this at the thought of part of the MN falling to Germany, just imagine the reaction to the idea of the most powerful navy in the world falling into German hands...!!
Actually, the US buildup was a reaction to fear that Britain would fall shortly after France did and that the British fleet would fall into German hands along with the French one.
It doesnt really matter if they think Britain will collapse or not, or if help they give is wasted, they simply dont have a choice but to help. national interest demands it.
I already responded to this one. Of course the US had a choice. They could either use the vast majority of the US's limited production to build up US armed forces or they could send a large hunk of that production to shore up Britain. If Britain was going to fall anyway, building up US forces made more sense, because anything sent to Britain would be lost when Britain fell. Sending arms to Britain only made sense if there was a substantial chance that those arms would tip the balance and either prevent or substantially delay the fall of Britain. Historically Roosevelt gambled that they would, and history proved him right.