Reread your history... FDR provided a hell of a lot of covert support for Britain before getting official congressional support. Cut off the flow of supplies from the US, and see how long Britain would last alone.
As it was, the primary reason Britain won the Battle of Britain was due to Germany shifting focus from airfields, radars, and factories to London hoping to break morale. At the time they switched strategies there were only a handful of Spitfires and Hurricanes left. British production could not match the the Luftwaffe when they were using the correct strategy. Thank Hitler and Goering for being idiots.
If Britain was so great by itself, why did it retreat from Europe, China, and almost Africa? Britain was fighting a delaying action while the American public was being swayed toward being willing to fight another European war it didn't want. If Russia and Germany had waited a little longer before going at it, I am not sure what would have happened. But both FDR and Churchill knew what was needed to win the war: our materials and factories that were beyond the reach of either Germany or Japan. Only Japanese carriers and German subs really presented any threat to our sea power. Count the Japanese carriers built after Pearl Harbor, then count the US carriers built afterwards. Of course it was the British capture of Enigma, the German policy of reporting submarine positions prior to attack, and huge convoys complete with escort carriers that pretty much ended the submarine war in the Atlantic.
If you take away all the convoy support to Russia and Britain (ships, planes, ammunition, etc.), I believe Britain would have collapsed. If Russia had to face Germany alone and Japan at the same time without our supplies, I am sure even their massive resources would have been tapped out.
Even with our full support, it took a long time to overcome the Germans. If the Germans and Japanese had made better decisions, We might not have won at all. It took all of us to win. I will not even listen to any claim that the war was already won or could have been won without the US. Without us, I am certain the outcome would have been vastly different.
I don't ever mention the Italians, because I have never studied their contributions and failures much. I don't want to insult them based on heresay. Though I do know they had trouble defeating Ethiopia when they had armor divisions and Ethiopia barely had rifles and horses. But that is little different than the US in Vietnam or the USSR in Afghanistan, numbers and technology do not win a guerilla war.