This will be a short answer as I don't have time now for a full exploration of the actual causes of the Purges, a topic that remains contentious today. But what we can be sure of is simple: there was no immediate threat to Stalin's position.

In the realm of high politics, Stalin was secure as the unquestioned leader of the Soviet Union. It wasn't quite the absolute autocracy that would follow the Purges but Stalin had proven to be remarkably effective in welding together a strong team of like-minded industrialisers. Historians used to talk of a 'liberal opposition' centred around Kirov (eg Cohen's Bukharin) but that's no longer sustainable. The Left and Right Oppositions had been vanquished; in contrast to those the quashing of the Ryutin Platform (1932) was an almost trivial affair. Stalin was undisputed master of the Politburo and party apparatus.

That's not to say that there weren't tensions within the ruling elite or alternative nodes of power. Both the Red Army and regional party apparatus expected to have a say in economic decisions. But these did not constitute a serious internal challenge to Stalin's personal leadership.

Ditto if one looks outside the elite. The direction of the violence was directed largely downwards, the 'mass operations' killing hundreds of thousands of unfortunates. Yet there's no evidence that the elaborate conspiracies 'uncovered' by the NKVD actually existed. It strains credibility to believe that the USSR was criss-crossed with extensive paramilitary networks, coordinated by foreign powers and that just happened to be all discovered in 1937.

Instead the Soviet leadership looked around and saw large populations that it itself had created and categorised - ex-kulaks, recidivists, socially harmful elements, etc. But there's nothing to suggest a real threat to the state from these already repressed populations. Ditto with the national minorities who were targeted in the 'national operations'.

I personally don't see a credible threat to either Stalin or Communist rule in the 1930s. That's not to say that there wasn't reason for the atmosphere of quasi-panic that engulfed the leadership but the threat was surely not real.

Other questions:

• That's been the standard Stalinist line. Molotov in his old age admitted the absurdity of the show trial charges but excused them on the basis of security. That's unjustified paranoia. There's nothing to suggest that Stalin's political rivals would not have fallen in behind him in the event of a German invasion - an existential threat to the entire Soviet project. (Bukharin had been warning about Hitler for years before his death.) Not unless they were actually German spies all along. Hint: they weren't.

• Someone else can chip in on the military. What I'll note is that the Red Army possessed a pretty modern doctrine in the late 1930s. Glantz notes that, "In the mid-1930s, the Soviet Union led the world in production, planning, and fielding of mechanized forces. Perhaps most important, the Red Army was well ahead of its German counterparts in both theoretical concepts and practical experience of mechanized warfare."

The lead advocate of this modern approach to warfare? Marshal Mikhail Tukhachevsky, executed in 1938 as part of the purge of the Red Army. Sweeping executions devastated the army to the degree that ranking Soviet officers entered the war entirely inexperienced for their roles: many were unable to execute basic manoeuvres, never mind ambitious mobile tactics.

Sources

Getty's Road to Terror is probably the best place to start for the political manoeuvrings before and during the purges.

Hagenloh takes a very different approach in Stalin's Police, placing the terror in the context of a police force increasingly used to mass campaigns against broad sections of the population.

Glatz's When Titan's Clashed is the place to start for the Red Army's preparations for the war. And Dee's Molotov Remembers is, obviously, the source for the Molotov apologism.