Hitler invaded a foreign country. Like the Americans he tried to impose his will on the people of another country.
England had a treaty with the government of Poland so came, invited, to its assistance. England didn't try to overthrow the Polish government. Hitler did.
It wasn't England or America's place to overthrow the German, Italian, Russian or Japanese governments in WW2 until those countries attacked other soverign states.
That is completely different to going to another country to impose regime change. America had no right to overthrow Gadaffi or Assad while these guys stay in their own borders.
