Middle eastern countries were meaningless then since there was no use for oil, countries like india with minerals and crops etc were much more valuable!
The british empire was the largest ever, it did however cover a lot of large empty places: australia, canada, america at first and large parts of africa, the jewel in the crown as it is said was india, rich in mirearals, many crops which could not be grown else where etc, however Britain also controled some very strategic places e.g. the cape and the suez canal, infact the defeat of nepoleon led to a lot of french assets being repossed by the british. and aftre the fall of france there really wasn't any rival powers. Britain had entered the industrial revolution and were the first to produce the gun, the steam engine and made much advancement in naval warefare, by this time america had gained independence but britain remained the controllers of all world trade, with the new technology britain had policies such as to ensure a navy atleast twice the size of it's largest two rivals put together, hence britain, however much land it owned/didnt own, had the largest global conrtol ever because it owned the seas.
Back to the question: economic collapse in america looks like the largest threat but if this does not occur, if america just carries on as it is, war will not end it nor will any nuclear threat. The most likely cause will be that america is simply superseeded. Most people think the next super power will be China simply because of their high growth rates, but china is very isolationist and this is not asomehting startded after ww2, historically china was one of the first civilized nations, sometime during the first dynasty the king sent out explorers to all the corners of the earth but all they found was nomads and barbarians(mabe somehting in egypt) so traditionally chinese culture looks in on itself as the center of the world and although it has a large amount of exports china seems not to have a desire to look outside it's self or a desire for it's own expansion to the outside world. The next super power could very well be india, it is a huge country reletively new in terms of independance and organisation it is still growing for the first time, rich in people, land and many assets e.g. gold, valuable stones etc so india has a good chance
