These countries, if they were determined to fight, seemed basically impossible to defeat. Russia had masses of people and land. It would not have been physically possible for German, with its limited population, to conquer and occupy the entire country.
Similarly, even if Germany did manage to invade England, the British empire still had many parts of the world they could fall back to.
It may help to answer this question if we knew what war you were speaking of :) WWI or WWII, I presume?
For starters, paid labor > slave labor for multiple reasons but we all know why. If the Nazis had skipped the whole racial genocide business manpower and production numbers as well as quality of equipment would have been much more satisfactory for the Germans. Plus there's the citizens like in Ukraine who would have been on the side of the Germans until the Einstatzgruppen turned them into partisans.so
Its not clear Hitler wanted to invade England at all it may have been all talk. Not to mention it would have been nearly impossible as Germany's navy was nowhere near capable of mounting amphibious operations. Then of course there was the Royal Navy, the largest in the world at the time
The most successful path to victory the Germans could have undertaken to defeating both England and the Soviets was ignored and bypassed by an impatient and ignorant Hitler. This path I'm thinking of was the Mediterranean.
Nationalist Spain's refusal to join the war and assist Germany infuriated Hitler because he expected support after he lended help to Franco in the Spanish Civil War, but Spain was war torn and in no way wanted another conflict with anybody. But its curious Hitler never made any moves against the strait of Gibraltar, a narrow choke point separating the Mediterranean from the Atlantic ocean controlled by a small British force.
Then of course there was dropping paratroopers on Crete instead of Malta. Moving into Yugoslavia and Greece was already a wasteful unnecessary drain on German forces, but capturing an island with no real strategic value instead of an island that was smack in the middle of a vital area was a huge missed chance.
Then lets not forget the Suez Canal, and how small of an effort was made to capture it. Considering the amount of success Erwin Rommel was capable of accomplishing with a Corps sized fighting force, its not outrageous to assume that with an army sized force consisting of three or four Corps could have captured Cairo, Suez, Jerusalem Bagdad, Syria, Mecca, Kuwait, Iran...
If all that was accomplished before an invasion of the Soviet Union, the Germans could have been on the back door of Southern Russia's oilfields and Stalingrad from the start of the war.
Couple this situation with maybe an increased U-Boat blockade, one that was unhindered by not declaring war on the United States on the basis and Germany could have had a starving England. Churchill himself said the Uboats came closer to defeating England than Hitler's panzers ever did.
Who knows