Personally I feel this is a loaded question, as even in the summer of 1939 the German High Command realised that unless the Wehrmacht could achieve quick decisive victories in France and Poland the German army would be unable to fight a protracted war against the France/British alliance and the Soviet Union.
While the German military were able to achieve quick land victories in both countries, the Germans failed to ensure that the British, French, Poles, and other occupied countries did not evacuate their fighting men to Britain in time for the Battle of Britain. Prior to the Battle of Britain the RAF lacked enough experienced fighters to take on the Luftwaffe, and the arrival of Czech and Polish fighters who had fought the Luftwaffe in Poland helped tip the balance for the beleaguered RAF.
Once the Germans cancelled operation Sealion Hitler still thought he would be able to force the British to the the negotiating table, as until the Americans were able to enter the war there was little the British could do to counter the Wehrmacht aside from fight counter rearguard actions in Greece and North Africa.
There were four key moments in 1941 and 1942 that ultimately led to the defeat of the Nazi regime, though prior to life of the Siege of Stalingrad in February 1943 there was still a significant chance that the Germans would have been able to gain a decisive victory over the Russians.
The first key event was the Battle of Moscow from October 1941 to January 1942, which was the ultimate high watermark of the German Reich. Prior to the battle the Germans had only known victory, and while it would be another year before the Wehrmacht would be in full retreat, after the Battle of Moscow the Germans would never again have the full military advantage over the Soviets or the other allied forces.
The second factor was the attack on Pearl Harbour on the 7th December 1941, which dragged the Germans into an outright war with the USA, much to the chagrin of the Nazi High Command who had no desire to fight a war on multiple fronts.
The third factor, as mentioned above, with the German capitulation at the end of the siege of Stalingrad in February 1943, and the complete destruction of the German Sixth Army. From this point on the Germans were forever on the back foot, though they fought a dogged rearguard action all the way back to Berlin over the next two years.
The forth and final factor is that it was not until the Battle of El Alamein in November 1942 that the British were able to stem the tide of defeat and push back the German advances into British and French territory. Churchill famously described it as "...before El Alamein we only knew defeat, after El Alamein we only knew victory." The battle was won by a combination of Field Marshal Montgomary's tactical brilliance and the Royal Navy/Royal Airforce destruction of German fuel tankers carrying fuel for the panzer divisions in north Africa, helped in part by intelligence interceptions of German fleet movements.
So, in answer to your question:
It was clear from the commencement of hostilities in 1939 that if the Germans could achieve quick and decisive victories over the Poles, French, and Russians then the British would have to sue for peace before the Americans had a chance to enter the war. The means that in the period August 1939 and November 1941 there was a distinct chance that Germany would have been able to achieve a decisive victory over both the Soviet Union and the French that would force Britain to the negotiating table without the need for a costly and bloody invasion of the British Isles.
The Germans lost the Russian and north African campaigns because they were unable to effectively supply their forces with enough fuel and suitable clothing for the extreme conditions (particularly the Russian winter), which meant that while the Wehrmacht was the best trained army in the theaters it fought in, when faced with an enemy who could absorb the punishing losses dealt to it, and then counter attack against the German advance, the Wehrmacht were forced onto the defensive, and ultimately into surrender. Logistics was an art of war that the German High Command was never able to square, and even in the Battle of France you see this when the German Panzer divisions were forced to stop for resupply around Dunkirk (though this is disputed by some historians).
Sources
Polish fighters in the Battle of Britain
Destiny in the Desert: The road to El Alamein by Jonathan Dimbleby
Enemy at the Gates by William Craig
The Battle for Moscow by David Stahel
This is a pretty broad question so I'll stick to the part I know the most about. There was never any realistic possibility that Japan would defeat the United States militarily in WW2. As long as the United States didn't lose the political will to fight the disparity in resources and economic power was simply too great.
I know that this subreddit generally frowns on the use of websites as sources, but since combinedfleet.com was created by Jonathan Parshall and Anthony Tully (the authors of Shattered Sword) I hope they'll make an exception. There's an article on there called Grim Economic Realities which shows that even if the USN had suffered a catastrophic defeat at Midway they would still have had a decisive edge in ships and aircraft over Japan by the beginning of 1944.
Since Japan was bound to lose in the long run would it have been possible for them to land a knockout blow early in the war? Again, the answer is no. Combinedfleet has another article about a potential Japanese invasion of Hawaii, and even in the dark days of late 1941 and early 1942 this would have been impossible for Japan. They simply did not have the logistical ability to move a large enough invasion force across the Pacific. Just getting Kido Butai in position for its raid on Pearl Harbor stretched their logistical capabilities to their limit. An invasion of Hawaii would have been on the same scale as Operation Forager (the US invasion of the Marianas) and even the US with all of its economic might couldn't pull that off until mid-1944.
It's not like the disparity in resources was some great surprise to any of the leaders in WW2. Churchill famously said that he "slept the sleep of the saved and thankful" after Pearl Harbor because he knew that once the full weight of US industrial power was on the side of the allies the ultimate outcome of the war was certain. Admiral Yamamoto knew this perfectly well as well, and prior to the start of the war he was an outspoken opponent of getting into a fight with the United States. One thing that I recently learned from Pacific Crucible is that Yamamoto pushed for a negotiated end to the war after the surrender of Singapore to Japan. He knew what the outcome of a prolonged fight would be, but he hoped that after its initial successes Japan could negotiate from a position of strength and settle the war in a favorable way. The irony of course is that the attack on Pearl Harbor that Yamamoto championed made it certain that the United States would never be open to a negotiated peace settlement and would only accept unconditional surrender.
Looked like to whom? To us now? To political or military leaders?