Is what it says on the tin, what if Germany remained on the defensive on the west and focused on Russia. Tensions between the USA and British Empire increase for whatever reason so you two major separate wars. What could be the implications of this?
One immediate consequence is that Britain does not seize the Turkish battleships, which
may lead to Turkey remaining neutral. (If not pursued by the RN, where does
Goeben flee to? Probably Austria.)
France cannot blockade Germany. France can block the English Channel, but not passage north of Scotland. German shipping in the Atlantic would be subject to French raiders, but not shut down. Also, France unlike Britain would not be able to impose restrictions on shipping to neutral countries that happen to border Germany (Sweden, Denmark, Netherlands). And of course Germany can trade at will with Britain, as can France.
As to the US-UK war, it's
extremely improbable, but for a thought experiment, suppose it breaks out in in 1915.
First, neither country is prepared for a major land war.
Second, while Britain has a larger navy, the initial area of operations will be near the US, which is a major offsetting advantage to the USN. Britain does have advance bases near the US: Halifax, Bermuda, the Bahamas, the British West Indies. The US has outlying bases in the Caribbean (Cuba, Puerto Rico, Panama). The US is also occupying Haiti and Nicaragua.
Course of the war: the US would occupy the British West Indies early on, but the effort would leave the US unable to attack Canada; the RN would move in strength to the Maritimes while the "BEF" would go to Quebec and Ontario. There would be major naval conflict. Britain IMHO cannot project its power into American waters, but would hold against American attacks. Britain could move to recapture Bermuda and perhaps the Bahamas - or British Guiana, which is farthest from the US.
Once both wars are in full swing, neutral ships will lead an exciting life, depending on where they are sailing from and to. Britain can try to interdict traffic to the US in the Atlantic, but I don't see that even the RN can interdict the US west coast, or traffic via Panama to the Gulf coast.
There's also French-flag traffic between France and its Caribbean colonies - which could cover traffic with the US. Dutch traffic with their colonies has similar possibilities. In the Pacific, does Britain try to interdict
Russian-flag traffic with the US West Coast?
(In all of this, it's an open question as to who would be shipping what to whom. It's not obvious what the US would particularly need to import or want to export.)
The US can raid British commerce all over the world - well, not in the Mediterranean or the Indian Ocean, but anywhere in the Atlantic or Pacific. But the US would not dare touch neutral ships, even if bound to/from British ports.
The US with 92M people outnumbers Britain and all the "white dominions". ISTM that the US can raise and equip more troops faster that Britain can raise equip,
and transport troops to Canada. US troops will mostly be raw - but Britain's new formations will also be raw (think "Kitchener battalions").
Britain can draw on India for troops, but will be reluctant to do so - except perhaps for use against the Philippines. With Malaya, Sarawak, Hong Kong, and Australia to base from, Britain can take the Philippines. Guam, Wake Island, and Samoa are toast.
Umm. One other point. The naval campaign in the western Atlantic would be the first to make use of aerial reconnaissance, mainly by airships (air
planes of the period lacking the range or endurance for maritime patrol). The US thus might have a major advantage in the only major supply of helium.