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Stalin's Problem. Soviet Russia has two objectives in the Balkans. One is to protect herself from German invasion. If she could advance her frontier from the Pruth to the summit of the Carpathians she could deprive Germany of one jumping-off place into the Ukraine. Since the danger would remain of this line being outflanked by a German advance southeastward from Poland, the risk of war with Germany in order to seize the Carpathians is probably not worth it in Joseph Stalin's calculating mind.
Russia's other Balkan object is age-old: an exit from the Black Sea. Russia's only other outlets to the world are through Vladivostok, the Baltic and the Arctic Ocean. The U. S. would be in a similar position if its only outlets to the world were through Alaska, Hudson Bay, Newfoundland, and the mouth of the Mississippi, which was held by a foreign power (the Turks). Since the 18th Century the Russians have hankered to possess the Bosporus and Dardanelles. When they tried to get them in 1854 the British, the French and later the Italians joined the Turks rather than let the Russians obtain a foothold in the Mediterranean. This campaign (the Crimean War) produced the Charge of the Light Brigade, Florence Nightingale and the beginnings of the Red Cross but left Russia still landlocked.
For Stalin to have Turkey, a secondary power, control the Bosporus is bad enough, but to have one of his rival dictators, Hitler or Mussolini, dominate the Straits would place him in an economic strait jacket. With them around, his chance of getting it for himself is small but he has every reason for cooperating with Turkey and Bulgaria to keep his rivals out, chiefly by lending the use of his Black Sea Fleet based on Nikolaev and Sevastopol. If Hitler and Mussolini are seriously weakened so that he does not have to fear war with them, he might well attempt to extend his control down the west shore of the Black Sea, but that is an opportunity he can only wait and hope for. His major problem at present is defensive.
Mussolini's Problem. With Hitler already in a political and military position to dominate the plain of Hungary and Stalin in possession of the Ukraine, Benito Mussolini can hardly pretend to be one of the big three dictators unless he controls at least a part of Europe's granary. Furthermore, Italy has not adequate food supplies at home and has for years depended on maize from Hungary to help feed her people.
The dictators may be friends but their deals are of a kind the U. S. Securities and Exchange Commission would approve—at arm's length. Not only because of Hitler's stronger army did Mussolini have to yield dominance of the Hungarian Plain to Germany. The Dinaric Alps which cut off that plain from the Adriatic Sea and Mussolini are not the highest mountains in Europe but some of the most rugged and impassable. The Pindus Mountains in Albania and northern Greece are considerably higher (they include Mt. Olympus, 9,730 ft., with Ossa and Pelion handy for climbing) but much easier to traverse with an army. There are actually two or three roads through them.