Paul Kennedy offers us an outstanding historical analysis. He looks at how appeasement has been used and appeared in various times; that it has not always been a bad thing, opprobrium to be hurled at one's opponents, but rather a useful tactic to keep one's nation focused on its priorities.
Just as importantly, he sets out how the world must have looked to British statesmen at the end of the 1930s, and their choices. History, in retrospect, looks different than does the current situation, because in reading history we know how things turned out. This is why the comparisons of Iran (or wherever) to Nazi Germany are foolishly overbroad. We don't know how it turns out with Iran. And how it turns out also depends on what we do or don't do, but we don't know how much.
There's a lot there. Read the whole thing.
And a couple of relevant bits and pieces:
On Iran, by Trita Parsi
And China, by Jamie K.