"I'm assuming this is simply an attack on the Commmonwealth realm? So UK, Canada, Australia and New Zealand and a bunch of Caribbean islands."
Well the Commonwealth is a lot larger than that - India, Pakistan, Malaysia, South Africa, Nigeria, Kenya etc etc. In a full scale Plan Red scenario that adds up to a lot of opponents for the US. Not many of them would be in a position to actively strike US soil but they would still need to be considered. Of course this contributes to the whole ASBish nature of the idea.
"Nations do not declare war on their cordial friends and close trading partners of more than 100 years of uninterrupted peace and alliance without a damn good reason to do it."
Exactly, which is why this scenario no longer makes sense. The hows and whys leading to this possible modern day Plan Red are unknown, to talk about how the world or the US public respond is pure conjecture because it would depend on the unknown causes.
With all those unknowns taken into account, and suspending disbelief in such an unlikely event, I think the scenario would play out with;
i) US occupying Canada, Carribean islands and other small British, lightly defended, territories either near US territory or of some strategic worth (e.g. Ascension, Samoa).
ii) Resistance in Canada leads to a brutal ongoing insurgency in Canada with attacks made in the US.
iii) US bases in any Commonwealth country are quickly overrun. US forces stationed in neutral countries leave. Afghanistan becomes an even bigger mess as all western troops get recalled, probable reversion to warlordism with the Taliban increasing control over time.
iv) US controls the seas and airspace anywhere it is serious about. This allows the US to neutralize strategically important targets (airbases, ports etc) in the UK, India, Australia etc. However, the US lacks the planes now to control the skies everywhere simultaneously, so counterstrikes by UK and allied forces are still possible.
v) US army is too small to conquer much else so the war reaches strategic stalemate, assuming a non-nuclear conflict. International trade plummets, UK and US economies stagnate and then crash due to closure of traditional trade routes (even US shipping will suffer serious losses) and escalating tit-for-tat economic warfare. Cyber warfare becomes increasingly important, with boundaries being pushed as to what is acceptable (e.g. should we target an enemy's utilities if that risks newborn babies in hospitals? Just how far would you go to 'strike back' if the situation gets desperate?). Standards of living and social cohesion in US/UK/Aus/NZ all suffer. India's electrical system is destroyed by the US, although this could almost happen anyway without US involvement. Other British allies suffer too but probably to a lesser degree as they are on the periphery.
vi) Rest of the world concludes we are idiots. So does most of the populations of the involved countries, social unrest goes through the roof and if any of these countries hold democratic elections then there will be changes in government. It takes decades to repair the economic damage to both sides. US is finished as the global power as no one trusts them (an unremarked but vital requirement for being the global superpower).
Basically, no one's a winner, except perhaps China. and Russia.