We had a powerful Navy but a small standing Army so we were roughly evenly matched.
In fact it was the French who you should thank.
In the First world war without American involvement it would of been a draw which would of been a good thing as then the nazis would never have risen to power. Britain was never under threat
As regards the Second World War
By the time America entered the war the threat of invasion to Britain had long passed.
And if in 1940 the Germans had carried out operation Sea Lion (code name for the invasion) there was a chance it would not work. The German Navy was not powerful enough to land a big enough force and logistics.
The German military was broken in the East, Battle of Moscow , Battle of Stalingrad and Kursk.
Germany new they were losing the war by summer 1943.
If America had not entered the war the most likely outcome would of been A Soviet Dominated Europe by 1947 unless the German officer corp. over threw Hitler, and came to an understanding with Churchill and Stalin, which would of been very likely as with out FDRs backing Churchill may not of pushed for total capitulation and Stalin would have been happy to make a deal
ps
Also its about time you Americans thanked us for holding the line in 1940 and 41 giving you breathing space to rearm.
PPS
Mel Gibsons Patriot. Most favorites bit when the evil British colonel who was wounded was jumped by Mel Gibsons priggish self-righteous all American hero son, and the son was got a sword run through him. My mates and me cheered that scene.
Cheers Tony