Albert Speer recalled after the war that President von Hindenburg asked Adolf Hitler never to enter into an alliance with Italy.
The old man was right, of course. The alliance with Italy cost the Germans dear. Having to intervene to rescue the Italians after they invaded Greece delayed the invasion of the USSR and may have lost Germany the war.
Italy soon came to regret going to war with the Allies in 1940 (taking Nice and Savoy was the motive).
Mussolini ended up dismissed in 1943 and shot in 1945. Had he remained non-belligerent he'd probably have been in power till his death, like Franco, and Italy would have been spared a calamitous invasion.
(Fascists would add that she would have been spared Americanisation, but Italy has not really been Americanised.)
For Romania the consequences of Italian entry into the war were as calamitous. Had Italy stayed out, England and America might well have invaded the continent via the Balkans (Yugoslavia, to be exact) and Romania and Bulgaria might have been spared Communism.
Hitler up to 1937 wanted England as an ally, but that was impossible because of ideological differences, on England's side, and because (according to Richard Overy) we were not prepared to give Germany a free hand in the east.
Baldwin in 1936, on the other hand, was in favour of letting Germany attack the Soviet union and so was his successor Neville Chamberlain. Somehow this was lost sight of and Chamberlain ended up committed to Czechoslovakia, Poland and Romania.
To use an analogy from the game of rugby, Chamberlain and the Foreign Secretary the Earl of Halifax fumbled the ball.
Hitler also hoped up to 31 March 1939, when England gave her fateful guarantee to Poland, for an alliance with Poland. That, surprisingly, would have made sense. Had it happened the German and Polish armies might have marched in a victory parade in Moscow. Instead, both countries were devastated.
Germany would have invaded the Soviet Union at some point in any likely scenario, so long as Hitler remained in power.