What was Hitler's back up plan if the The Molotov–Ribbentrop Pact negotiations had failed to deliver any agreement?

Greetings! I shall attempt to answer your question(s) by instead answering another one: Why was the Molotov-Ribbentrop Pact guaranteed to succeed? The rationale behind this approach is twofold: the original question posed is actually rather counterfactual and borders on pure fantastical "what-ifs?" (which both this sub and academic historical studies frown deeply upon). Further, in order to understand why Hitler had no backup plans (at least none that we have on record), we must understand why both he and Stalin had it in their best interests to sign the Molotov-Ribbentrop Pact. So with this preamble aside, let us begin.

Firstly, the Molotov-Ribbentrop Pact perfectly suited the foreign policies of both the USSR and Nazi Germany. Hitler had already aggravated the rest of Continental Europe (France and Britain in particular) after occupying the rest of Czechoslovakia in March 1939, and Stalin himself was not particularly favoured by the German Chancellor, whose Anti-Bolshevism and vehement Anti-Socialist Nazi ideology were well known to all. However, as Hitler's expansionist plans turned towards Poland and Chamberlain's government guaranteed Polish independence on March 31st, it became clear to both dictators that Realpolitik was the better option to ideological purity.^(1)

Even moreso for Stalin, the Moscow Mission by Britain and France in August 1939 failed spectacularly; with negotiations on a proposed Anglo-Franco-Soviet Alliance getting nowhere. The problems were apparent from the outset. Here's historian Michael Jabara Carley on the matter:

"The talks seemed of so little import that Halifax [British Secretary of State for Foreign Affairs] had 'scarcely perused' British instructions. Concerns about Soviet impatience if the British dragged out the talks were shrugged off. And the British delegation was instructed to avoid discussion of Soviet aid to Poland and Romania; the Soviet Union would have to negotiate directly with the Polish and Romanian governments".^([2])

The key problem was that France and Britain were both unwilling to accept Moscow's demands for an "ironclad military alliance".^([3]) Stalin wanted a conditional collective security agreement, in which the Anglo-French government could support the Soviet Union in the eventuality of German invasion (a most astute prediction). After the failures of these talks with the Western Allies, it appeared as though Stalin's best option (ironically) lay in making a deal with Adolf Hitler and Nazi Germany. In addition to the greater likelihood that Hitler's diplomats would actually take negotiations seriously, the Soviet leader believed that neutralising the German threat directly would not only secure his country's precarious military isolation, but also give the Red Army precious time to recover after the disastrous "Great Purges" which had decimated its senior command. Here's Stalin himself actually echoing this decision in a public address to the Soviet people after Operation Barbarossa (dated July 3rd, 1941).

"What did we gain by concluding the non-aggression pact with Germany? We secured for our country peace for a year and a half and the opportunity of preparing its forces to repulse fascist Germany should [it] risk an attack on our country despite the pact."^([4])

Hitler also had reason to conduct this surprising bit of foreign diplomacy: it would secure his Eastern front, leaving his armed forces with more security whilst they dealt with the Western threat of Britain and France. Further, Hitler's own doubts about the reliability of his Japanese counterparts in occupying the Soviets from the Far East had increased after the Japanese defeat in the Khalkin Gol border conflicts during 1939. Here's historian Owen Matthews on the subject:

"Hitler had once hoped the Pact of Steel with Japan, threatening Russia's own Far Eastern provinces, would be enough to keep Stalin in check. But the Japanese failure at Nomonhan [Khalkin Gol] convinced Hitler that more direct measures were needed to keep Stalin out of the war - namely, a direct deal with his counterpart in the Kremlin."^([5])

It was in this light that the two countries, once considered polar opposites ideologically and politically, began their secret negotiations for a non-aggression pact. It was also therefore, the reason why the Molotov-Ribbentrop Pact was due to succeed from the start, and hence why Hitler had discussed no "backup plans" prior to authorising the idea of a deal with Stalin. Here's the German dictator himself addressing his generals on the shocking decision, dated August 22nd, 1939 (just two days before the Pact was signed):

"I have decided to go with Stalin... On the whole, there are only three great statesman in the world: Stalin, myself, and Mussolini."^([6])

On a sidenote, it might interest you to know that Stalin had a backup plan, in the sense that the Soviet Union had some form of military arrangements incase a pre-emptive invasion of Germany was required. This was Operation Groza (thunderstorm), a plan which was signed before Operation Barbarossa actually had been, on September 18th, 1940. The plan was ambitious to say the least, and Matthews paints the numbers most vividly.

"It detailed a Soviet offensive across Poland to Berlin and beyond. The plan of Operation Groza listed the military forces available to Stalin as 300 divisions, including 8 million soldiers, 27,500 tanks and 32,628 aeroplanes."^([7])

So that concludes what I hope has been a somewhat enlightening response to your answer, without indulging in the dangerously speculative "what if" of a non-existent Molotov-Ribbentrop Pact. I have attached alongside the sources below some further reading on the Pact's creation, and an overview of historiography regarding Stalin in particular (as Soviet and even Russian academia have been waging war with other nations' historians to portray the dictator as off-guard to the betrayal of the Nazis). Feel free to ask any follow-up questions on this most interesting of diplomatic history topics, and happy reading!

Sources:

[1]: Matthews, Owen. AN IMPECCABLE SPY: Richard Sorge, Stalin's Master Agent. S.l.: Bloomsbury Publishing, 2020.

[2]: Carley, Michael Jabara. "End of the 'Low, Dishonest Decade': Failure of the Anglo-Franco-Soviet Alliance in 1939." Europe-Asia Studies 45, no. 2 (1993): 303-41. Accessed December 29, 2020. Accessed here (free).

[3]: Carley, "End of the 'Low, Dishonest Decade': Failure of the Anglo-Franco-Soviet Alliance in 1939."

[4]: Quoted in Roberts, Geoffrey. "Stalin, the Pact with Nazi Germany, and the Origins of Postwar Soviet Diplomatic Historiography." Journal of Cold War Studies 4, no. 4 (2002): 93-103. Accessed December 29, 2020. doi:10.2307/26925240. Originally taken from “Soviet Documents on Foreign Policy: Volume III. 1933–1941. Edited by Jane Degras. (London, New York, Toronto: Oxford University Press).

[5]: Matthews. AN IMPECCABLE SPY: Richard Sorge, Stalin's Master Agent.

[6]: Nekrič Aleksandr M. Pariahs, Partners, Predators: German-Soviet Relations, 1922-1941. New York: Columbia Univ. Press, 1997.

[7]: Quoted in Matthews. AN IMPECCABLE SPY: Richard Sorge, Stalin's Master Agent. Originally found in the Russian archives.

Further Reading:

1: Uldricks, Teddy J. "Stalin and Nazi Germany." Slavic Review 36, no. 4 (1977): 599-603. Accessed December 29, 2020. doi:10.2307/2495264. (JSTOR required)

2: Uldricks, Teddy J. "The Icebreaker Controversy: Did Stalin Plan to Attack Hitler?" Slavic Review 58, no. 3 (1999): 626-43. Accessed December 29, 2020. doi:10.2307/2697571. (JSTOR required)

3: Gorodetsky, Gabriel. "The Impact of the Ribbentrop-Molotov Pact on the Course of Soviet Foreign Policy." Cahiers Du Monde Russe Et Soviétique 31, no. 1 (1990): 27-41. Accessed December 29, 2020. http://www.jstor.org/stable/20170697. (JSTOR required)

4: Moorhouse, Roger. The Devils' Alliance: Hitler's Pact with Stalin, 1939-1941.