I suspect that Hitler wrote to Mussolini exactly what Mussolini wanted to read. Comments like those need to be taken in the context of Italy's position vis a vis the British in the Mediterranean.
Theres no evidence to suggest that Hitler was not making honest comments to Mussolini either. And plenty of circumstantial evidence to say that he was being up front. if he was trying to win over or sooth the italians by that letter, it was a terrible miscalculation. many in the italian leadership never forgave the germans for their duplicity and disingenuous treatment of the italians (even Mussolini was insulted by it, though he was so beholden to hitler by then that his displeasure counted for nothing really) over the russian invasion. From Italy's POV it was a disaster on many levels, not least because it completely jeapardised their oil and grain imports from the Soviets.
None of the Nazi leadership had a very good grasp of foreign affairs. I don't believe that Hitler or any of his panjandrum had the faintest inkling of the real relationship between Britain and the USSR in the late 1930s.
This is also completely irelevant. Nazi decisions were seldom based on hard facts. hitler repeatedly and disastrously would rely on his "intuition" and "instincts" to guide his decisions about a whole range of issues. What was important isnt what was true, or what Hitler knew. It was what he believed, even if (as was often the case) such beliefs bordered on the delusional
I'd also like to see some evidence that Britain was working for this alliance with the USSR.
Why. its completely irrelevant what they were actually working on, or how successful they were. After the Soviet betrayal in 1939, relations between Britain and Russia were icy, to say the least. Thats not the issue. the issue was how hitler, and his cronies perceived the continued British resistance, and why they remained so intransigent and defiant. Hitlers letter to Mussolini provides a lot of evidence as to what hitler was thinking, and whilst that may be an inconvenient truth for those of us who choose to believe that Hitler was following some grand strategic plan, its a bit hard to try and subvert what many perceive of irefutable evidence to the contrary.
Despite the frozen relationship, Britain gave repeated warnings to the russians of the impending invasion, as far as british intelligence knew about it, and without compromising ULTRA. Stalin, like all the warnings he received, ignored these pieces of advice. There was no formal entente with the Russians until June 1942, but in reality the first British aid convoys started from september 1941 from memory. this is not the actions of a Britain indifferent to making some kind of collective security arrangement with the russians. if they didnt care, they wouldnt have bothered talking to them at all. but they did, and Hitler must have known at least some of those goings on i am sure. Being the paranoid delusional man that he was, what sort of conspiracy thinking would the following have on Hitler
1) Absolute defiance and intransigence by the British
2) Long term economic destruction frombombing and the blockade
3) Deep suspicion of a possible Soviet-British entente.
Hitler knew he had to deal with either Britain or russia to dominate Europe. He convinced himself that he couldnt control the British, was disdainful of the Soviets, and saw German strength in its land capability. these are the factors that drove hom toward an attack on the Russians, not some long term grand scheme that he had had since 1919.
We need to take a look at Soviet relations in the interwar year to gain some understanding as to why this notion that hitler "always wanted to attack the Soviets" is the total crock that it is.
The young Soviet Union struggled with foreign relations, being the first communist-run country in the world. The old great powers were not pleased to see the established world order rocked by an ideology claiming to be the harbinger of a world revolution. Indeed, many had actively opposed the very establishment of Soviet rule by meddling in the Russian Civil War. Slowly the international community had to accept, however, that the Soviet Union was there to stay. By 1933, France, Germany, Great Britain and Japan, along with many other countries had recognized the Soviet government and established diplomatic ties. On November 16, 1933, even the United States joined the list. Thus, by the 1930s, Soviet Russia was not quite the international pariah that it had been. And one of its earliest friends had been, you guessed it, germany, including the Nazis
In the immediate post war period after Versaille, the main enemy of the Soviets was in fact France. Franco-Soviet relations were initially hostile because the USSR officially opposed the World War I peace settlement of 1919 that France emphatically championed. While the Soviet Union was interested in conquering territories in Eastern Europe, France was determined to protect the fledgling nations there. This led to a rosy German–Soviet relationship in the 1920s. However, Adolf Hitler's foreign policy centered on a massive seizure of Eastern European though not directly or overtly Russian lands for Germany's own ends, and this caused a natural souring of Soviet-German relations. When Hitler pulled out of the World Disarmament Conference in Geneva in 1933, the threat hit home to both the USSR and France. Soviet Foreign Minister Maxim Litvinov reversed Soviet policy regarding the Paris Peace Settlement, leading to a Franco-Soviet rapprochement. In May 1935, the USSR concluded pacts of mutual assistance with France and Czechoslovakia; the Comintern was also instructed to form a united front with leftist parties against the forces of Fascism. The pact was undermined, however, by strong ideological hostility to the Soviet Union and the Comintern's new front in France, Poland's refusal to permit the Red Army on its soil, France's defensive military strategy, and a continuing Soviet interest in patching up relations with Germany all left the Soviets quite isolated. Germany under Hitler were keen to also seek rapprochment with the Soviets, as Hitlers immediate aims were not overtly aggressive toward the Soviets. He had other fish to fry....
The Soviet Union also supplied military aid to the Republicans in Spain, but held back somewhat, mostly to retain a reasonable relationship with the germans. Its support of the government also gave the Republicans a Communist taint in the eyes of anti-Bolsheviks in the UK and France, weakening the calls for Anglo-French intervention in the war.
Hitler actually feared the Soviets at this time, and in his own version of containment the Nazi government promulgated an Anti-Comintern Pact with Japan and later Italy and various Eastern European countries (such as Hungary), ostensibly to suppress Communist activity but more realistically to forge an alliance against the USSR. This shows the clear intent of the germans to forge a mass alliance against the Soviets,but Germany's own aggression and transgressiopns completely destroyed that idea beyond the abject german lackey states of eastern europe
When Nazi Germany entered Czechoslovakia, the Soviet Union's agreement with Czechoslovakia failed to amount to anything because of Poland and Romania's refusals to permit a Soviet intervention. Things were not going too well for the Russians in foreign relations, not least because getting into bed with the Russians was distinctly unheathy for their partners. Despite this on April 17, 1939, Stalin suggested a revived military alliance with the UK and France. The Anglo-French military mission sent in August, however, failed to impress Soviet officials; it was sent by a slow ocean-going ship and consisted of low-ranking officers who gave only vague details about their militaries.
However, this is all evidence that alliance between the west (which by late 1940 was reduced to just Britain and her empire) was a possibility if the right circumstances existed. this had to be known to the Germans, and given their experiences in WWI had to have been a significant influence in their decision to liquidate the USSR. However if such risk no longer existed because of a British capitulation, there is every reason to suggest the Germans would demoobilise (as they did partially after France) and not get into another or further wars for the time being.