We didn't need to get involved? We let the situation get so out of hand that Germany was a very real threat to our existence as an Empire and it cost millions of lives to stop them.
They would have stopped him, which would probably have been enough to bring him down given the nature of his regime. If we had stood up to him at any point from 1934 onwards the Second World War would almost certainly not have happened, certainly not
This is where you are going wrong.
The Germans had been secretly co-operating with the Soviets as part of the Rapallo Pact, but they were testing tactics and equipment that were at best equal to what the British and French were fielding, there was very little innovation beyond that (and what did happen was in the Soviet's benefit) and it was emphatically not about stockpiling equipment that could be quickly used (which is what "building up the German Army" would require). We even knew about it at the time, via our own intelligence and by reports in the German press, but decided to ignore it.
As an example of how "developed" they were, when Guderian was posted to be the expert on armoured warfare for the German Army in 1931 he had never actually been in a tank (he had to wait until a working holiday with his wife to Sweden, when the Swedes let him have a go on one of their ex-WW1 machines), and he and his then boss had to create the Panzer forces from nothing.
When Hitler took power in 1933 the Panzer forces didn't exist in any real form, nor did the Luftwaffe, nor did the Kriegsmarine. They were inferior to the French (never mind everyone else), which is why the French insisted at the Geneva Conference that Germany should remain militarily inferior to them. They were still inferior to them by 1936, when Hitler went into the Rhineland and terrified almost all of his senior officers who knew what would happen if the French sent their army in. If he had been confronted by a combination of the UK, the French, the Poles and the Czechs at any point up to the Anschluss (and probably any point up to Munich) he would have lost, or more accurately he would have backed down and then lost.
In short, the dominance they appeared to have in 1938/9 that you point to was because we had spent five or six years not stopping him, letting him build his forces up and generally looking the other way (edit: and not rearming properly ourselves). Had we stepped in earlier, as we might well have done (and as Barthou had tried to do before his death) then he would not have got to that point.
That is what I was trying to say when mentioning our inaction during the 19th century, during the 1930s, what it led to and the fundamental daftness of yet more inaction now.
Like Pete said, excellent post. I dont know a whole lot (bar the obvious) about the build up to ww2. Insightful stuff.