@yaxomoxay: You wrote of wondering why Halifax hadn't succeeded Chamberlain, in 1940.
I meant to return to this point, (about Halifax) but was waylaid while discussing both Churchill and Chamberlain (both of whom, in very different ways, merit respect), and subsequently forgot about it.
Re Lord Halifax, there seemed to have been a number of considerations in play: Firstly: The vacancy (in the position of PM) came about because of the need for the construction of a 'national government' - i.e. an all party administration, to manage the conduct of the war (as had happened in WW1) - and Labour (Attlee had been leading the party since 1935) had made it clear that they would not serve in such a government under Chamberlain.
The two Tory (Conservative) candidates deemed likely to succeed Chamberlain were Halifax (who was Foreign Secretary) and Churchill (at the Admiralty).
Of this pair, Halifax was undoubtedly the preferred choice of the establishment, i.e. the King, the Tory party, etc.
However, he was manifestly unsuited for the top position during war by temperament and altered political circumstances.
Politically, he would have been a difficult 'brand' to sell - especially in a time of war - because he was a member of the House of Lords, not the Commons; the days when a PM could expect to hold the reins of power, and control both Houses of Parliament from the elevated vantage afforded by membership of the Lords were long gone. By 1940, the PM had to come from the Commons.
(Bear in mind that years later, both Tony Benn, born as Anthony Wedgwood Benn, and Sir Alec Douglas-Home, who was to succeed Harold Macmillan as PM, both resigned their seats in the House of Lords to contest seats in the Commons.)
Temperamentally, he didn't want the job, not in those circumstances, (war, facing the very real possibility of defeat) not least because it seems (that privately) he thought that the government would fail, - perhaps, in circumstances of military defeat - and he might then be in a position to easily succeed the fallen Churchill, albeit under circumstances of compromised independence.
Labour, about to enter government, also had a veto (rather than an express choice, although, in fact, it almost amounted to the same thing) over the name of the person who was to serve as PM - in other words, as they had already made perfectly clear with regard to Chamberlain, they had the right to stipulate whom they would not serve under. While they remand silent on the internal Tory succession, I cannot imagine that they were other than lukewarm on the matter of serving under Halifax.
Moreover, - on the matter of temperament and core values - to me, (yes, with the pluperfect vision afforded by hindsight), it is also clearly worth noting that the right choice was made, in appointing Churchill PM, for, very shortly after that government was formed, with Churchill as PM, the inner war cabinet - Halifax was still Foreign Secretary and remained in that post until the following December when he was appointed Ambassador to the US, - discussed the idea of a negotiated peace with Germany.
This was at a time of crisis: It occurred during the Dunkirk evacuations, with France on the verge of complete defeat, and Italy - not yet in the war, and thus, for Halifax (keen to keep her out) - seen as a state mooted as a possible "honest broker" of such a negotiated peace, the conditions of which would have left Britain, in a position somewhat akin to Spain, i.e. left alone to mind their own business, while Germany remained in control of Europe.
The key point here is that - at the worst possible time - Halifax was prepared to contemplate a negotiated peace with Germany; it is not that he was a Nazi, or fascist, but that he wished to preserve the ordered, deferential, hierarchical world he knew (and loved) and was prepared to treat with monsters - monsters he didn't think could be withstood, let alone defeated - for this to be achieved.
Both Roy Jenkins's biography of Churchill, and Andrew Marr's (excellent, and well worth reading) "A History of Modern Britain" discuss this series of extraordinary meetings of the inner war cabinet, meetings where Churchill - disliked and distrusted by many in the Conservative party, newly appointed as PM, as yet unsure of his authority in the Commons or cabinet, equally unsure of how Labour (which had pursued a policy of pacifism not all that long before, even though Attlee himself - still referred to as Major Attlee - had served with distinction during WW1 - and which had loathed the economic policies of the Conservative party) might respond to this overture - and he knew that Halifax, still Foreign Secretary - who had been the preferred establishment candidate for PM, even though he had declined it - was strongly in favour of a negotiated peace, and had supported Chamberlain's policy of appeasement, while Chamberlain himself, the architect of appeasement, was still in government, and treated respectfully by Churchill, almost as a deputy PM, and - given his own earlier actions and position (appeasement) might also favour - as a reluctant necessity - the idea of a negotiated peace with Germany.
In those meetings, perhaps paradoxically, and rather unexpectedly, it was the two Labour members (Attlee and Greenwood) who passionately opposed the idea of a negotiated peace with Germany, - they rightly saw it as a death for democratic values - and insisted, instead, that the fight continue, along with Churchill, whose very instincts revolted at the idea, seeing it, correctly, as a form of future vassalage, "(probably) with someone such as Mosley as PM" (and, one suspects, Edward VIII back on the throne); the Liberal leader, Sinclair, concurred.
Likewise, it is now clear that Halifax was the most enthusiastic of those voices - exclusively Tory - who pushed for a negotiated peace.
Given his marked preferences on that key issue, (and his almost utter indifference to Europe, like Chamberlain, he was insular by inclination), I think it would have been nothing short of catastrophic - because this was an instance where character and core values and vision mattered, every bit as much as ability, or competence - had he been appointed PM in 1940.
In any case, - and I suspect mindful of those days in late May 1940 - Churchill took advantage of the death of the UK ambassador to the US later in the year, and appointed Halifax to succeed him, thereby getting him out of the way, into a position where he could do little harm (the US wasn't yet in the war) and some good, and also allow Churchill to appoint his own choice as Foreign Secretary, who happened to be Anthony Eden (who had a much better record on matters related to appeasement and how best to deal with Nazi Germany at such times; Eden's political problems lay in the future, with his marked tendency to mistake others such as Nasser - who merely wished to assert sovereignty - as future potential Hitlers, when they were nothing of the sort.)