Himmler was only eighteen when he joined the Freikorps Oberland in April of 1919 – and thus “got on the radar” of the Thule Society (this Freikorps was established by the latter, after all).
Why did young Heinrich Himmler decide to join Freikorps Oberland? First, he was still deeply hurt because he did not take part in the Great War – and thus did not fight (and very possibly, shed his blood) for his beloved Fatherland. Especially given that his older brother was a decorated war veteran – and an officer and his godfather fought and died heroically for his country.
The civil war in Bavaria, triggered by the establishment in Munich of the so-called Bavarian Soviet Republic (BSR) on April 6th, gave him the opportunity that he missed just a few months earlier.
Alas, he was denied this opportunity for the second time in a row. He did not take part in any action (possibly, he was deemed too young for that); instead spent all his time in Freikorps in a support/clerical/administrative position. It is possible that he demonstrated superb organizational and administrative skills and, as they say in such circumstances, got noticed.
It is also very likely, that the establishment of BSR planted in his heart, mind and soul the seeds of the concept of the “existential racial war with Judeo-Bolshevism” – and strengthened this idea in hearts, minds and souls of members of the Thule Society, The Org and The Black Sun Society.
After all, BSR was established and led by Jews (Ernst Toller, Gustav Landauer, rich Mühsam, Eugen Leviné, etc.). And these Jews – for all practical purposes – were the puppets of the Russian Bolsheviks (led also by Jews – Leon Trotsky, Grigori Zinoviev and many others).
Furthermore, Leviné and Co openly stated that Bavaria was to be but a springboard for the Bolshevization of Germany and the Europe (and then of the whole world, of course – as was initially planned by Marx et al.).
Why did Himmler join Freikorps Oberland and not another Freikorps? Well, there was no other Freikorps around so he really had no other option. Did he know that Freikorps Oberland was established by the (official) president of the Thule Society and hence by the latter? Almost definitely – the Thule Society was at the time not exactly a secret organization and was heavily involved in counter-revolutionary activities.
And for an obvious reason – BSR government (its political police, actually) accused the members of the Thule Society (probably correctly) of trying to infiltrate BSR leadership and of attempting a coup. On 26 April, the Communist secret police in Munich raided the society’s premises and took seven of its members into custody (actually, they were taken hostage).
Four days later, they were ruthlessly executed by a firing squad in reprisal for the counter-revolutionary activities of their alleged associates. Amongst the murdered in cold blood by the Reds (let’s call a spade a spade), were Walter Nauhaus (one of the official founders and leaders of the Thule Society) and four well-known aristocrats, including Countess Heila von Westarp who functioned as the (official) secretary of the Society, and Prince Gustav of Thurn and Taxis who was related to several European royal families.
Was Himmler interested in the occult and other activities of Thule Society (outside of politics, of course)? Probably not. At that time he was still a devout Catholic and was interested exclusively in joining a paramilitary unit and (ideally) in taking part in some serious action.
True, he was interested in the occult and in German paganism since (probably) his early teens, but at that time not nearly enough to join an occult group. Besides, it appears that he considered himself still way too young to take part in any such activities (but already old enough for the battlefield).
Hence, it was the Thule Society (and thus The Org and The Black Sun Society) who got interested (very interested, actually) in young Heinrich Himmler – not the other way around.