One of MacLean's earlier books, and perhaps stronger for being that. The action depends more on character and plans than coincidence or deus ex machina, which is more satisfying than some of his later books' plots. The characters are his usual types, almost superhuman, a man who drinks ridiculous quantities of alcohol without being affected, men who are willing to die but, generally do not have to; interestingly, there is a woman who functions as a romantic interest for the protagonist, which is fairly unusual for MacLean ~ perhaps he needed to put her in to further the plot, which her presence does do a few times, so overcame his usual reluctance for romance which might slow down the action.
Curiously, for a man who was concerned about keeping the action going, there is quite a large amount of moralising or philosophising in the novel, largely in the mouth of one character; certainly the character who expresses the opinions, which are essentially those which say all men are brothers and must learn to live together, is a man who is in a position to speak with authority: He had been a partisan during the Second World War, successfully and violently fighting the Russians in Ukraine, then ending up as an anti-government actor in post-War Hungary, helping his fellows to escape the torture and inhumanity of the Communist government. Unfortunately, i found that the philosophy slowed down the action at a point when really it should have been racing towards the end. Still, it seems to me that MacLean was willing to take this action because this character seems to speak for him, and he believed it essential that the world think about his points.
As mentioned above, the book takes place in Hungary; as it was released in 1959, it seems likely that it was inspired (some of the action makes this clear also) by the events of the Hungarian Revolution and its awful oppression by the USSR, and fairly clearly MacLean was so horrified by these actions that he found it necessary to use his available pulpit to publish his opinion of the Cold War.