This book is only around 200 pages long but expresses exactly what the author meant concisely and with style.
While it has been seen as a realist attack on idealism Carr actually sees realism and idealism as two concepts - the first epitomised by the bureaucrat who takes existing power structures into account in decision making and sees the differences between each case but has no interest in changing the system and no motivation to , the second epitomised by philosophers like Woodrow Wilson who are concerned only with the ideal they wish to achieve, treat every case the same as one to be changed to the ideal and don't pay enough attention to how they can get from the existing power structure to the new one they aspire to create.
Carr says pure realism would lead to stagnation with no-one having the motivation to make any change for better or worse - while pure idealism will always fail to achieve its aims because of its utopianism.
His solution is a balance between the two - having ideals to aim at but also taking into account existing power stuctures and thinking about how they can be changed to achieve ideals.
His analysis of the liberal theory of 'the harmony of interests' is interesting and pretty much an attempt to apply Marx's ideas of the 'false consciousness' of the 'proletariat' to international politics to explain why governments of countries harmed by the existing system often believe it is in their interests as much as the dominant states' interests - and why dominant states end up believing that what is in their interest is in every country's interest despite the inevitable conflicts of interest in reality.
The flaw in his argument is to personify states and assume that equality among states is the same as equality among individual people to the extent that he believed Germany and Italy should be allowed to have European and North African empires to match the British and French empires as a means of avoiding war.
This ignored the obvious unfairness and brutality of all empires towards the people of their colonies.
Carr's advocacy of a policy of appeasement also led to many shunning him at the time he wrote 'The Twenty Years' Crisis' and even today. To be fair to Carr the holocaust was not public knowledge in Britain in 1939. It's impossible to know what he would have written had he known about it - but very possible that it would have changed his mind.
His main point was that the status quo suited the countries which won the first world war and imposed an international system which benefitted them - free trade being 'the paradise of the economically strong' but not of countries with weaker economies which could not compete in it.
It might well be that if this unrestricted free trade regime hadn't been imposed the great depression and the surge in support for fascism and communism caused by mass unemployment could have been avoided.
Few if any people or books are perfect and 'The Twenty Years' Crisis' remains a masterpiece and relevant to this day despite any flaws in it or E.H. Carr.