It took the US to intervene for us to win the last war.
Not sure exactly what you mean there.
But as an observation, when the UK faced the very real threat of invasion in 1940 the US were no where to be seen. The German's needed air superiority to attempt a seaborne invasion and so the Battle of Britain was absolutely critical. In the end it was the RAF who won the battle (helped greatly by aircrew and commanders from Australia, NZ, Canada, RSA, Poland, Czechoslovakia, France...). Had the RAF lost the battle of Britain WWII may have taken a very different turn.
In 1940-41 when the German's threatened to take Egypt (which would have given them control of the Suez and opened up the middle east and it's rich oil reserves) it was the British, Australian, New Zealand, South African, Polish, Indian, Free French, Czechs and Rhodesians who stopped them. During this critical time the US were neutral.
If we look at D-Day and again we see that it was (despite what hollywood would have us believe) a primarily British operation. The planning for D-Day was carried out by British High Command whilst the invasion forces came under the direct command of Montgomery. The allied airforces and naval forces were also under British command.
In order for D-Day to succeed a number of things had to first happen:
- The German's had to believe that the invasion would occur at a time and place quite removed from reality. To achieve this the British SIS oversaw a highly effective deception plan. The deception plan was so successful that as late as July (1944) Hitler believed that the main invasion was still to come.
- The allies needed complete control of the channel. This was largely achieved by the RN with help from the Canadians. Indeed, 78% of warships and landing craft at D-Day were British or Canadian.
- The allies needed complete control of the air. This is where the US had the biggest military impact on the western front. The Luftwaffe had been degraded by the RAF over the course of 1940-43 but it was the USAF that really hurt the Germans in the lead up to D-Day (thanks largely to the Mustang P51D). Here it should be noted that the bulk of allied aircraft that took part in D-Day itself were from the RAF which included whole squadrons made up of Australians, Canadians as well as including Kiwis, Poles, Czechs and French)
- It was critical that German armour didnt engage allied troops in the opening days of the invasion. This was achieved by a variety of factors. The deception plan lead by the British SIS (MI6) played an important role. So too did RAF aircraft. But often overlooked is the important role played by the British SOE lead, trained and equipped French resistance.
- A huge factor in the allied victory in WWII was the British cracking of the German enigma code. The importance of this cannot be overstated for it allowed the Allies to know exactly what the Germans were saying. They knew where the Germans were deploying their troops, who was commanding these troops, what their positions were etc etc etc.
- The allies also needed access to a harbour in order that the invading forces could be reinforced and resupplied. This presented a problem as ports along the French coast were heavily defended. Enter the British who came up the the brilliant floating harbours called Mulberries. So effective were these floating harbours that they were still being used 10 months after the D-Day invasion.
- The British invented Bangalore torpedoes were also used very effectively to breach German barbed wire defences. They played a very important role at Omaha beach.
On D-Day some 140,000 allied troops landed on the 5 beaches. British and Canadian troops landed on three of these beaches and made up around 59% of the total seaborne invading force.
But when speaking of WWII most of the credit for defeating the Nazis belongs to the Soviets. Depending on what sources you choose to trust btw 82-91% of ALL German military casualties occurred fighting against the Soviets. At Stalingrad alone the Germans lost some 850,000 men. Without the Soviets the western allies could never have even contemplated an invasion of Nazi occupied Europe.
That said the Soviets almost certainly could not have defeated the Nazis without the help it received (directly and indirectly) from the Western Allies. So the defeat of the Nazis required a genuine international effort involving many countries. Each played their part.