It is easy to see how Eric Ambler came to enjoy so much success in Hollywood. His books have a cinematic quality to them. He is obsessed with advancing the plot. And he does so, here, with Background to Danger, in a fashion as strong as any other book he wrote, including A Coffin for Dimitrios. Ambler's chapters can easily be broken into scenes and you can practically see the shot as it would appear before the camera. Transitions, done with flashbacks, directly relate to cinema, as does his transitions that amount to plot points, along with easily visualized fades and dissolves.
Background to Danger was published a few years before A Coffin for Dimitrios. And it seems to me that Ambler is working towards what he will ultimately achieve in Dimitrios. That is, both these novels work towards what will be the "buddy movie" genre in Hollywood and both are fitted out in a manner that would later be seen in the anti-heroes of 70s films. Go to the film version of Dimitrios and this notion is even more evident, with two confirmed anti-heroes in the lead roles, Peter Lorre and Sydney Greenstreet as newly framed friends in the fight against corruption.
The unexpected friendship in Background to Danger comes with the relationship built up between Kenton ("not Kenten") and the Soviet spy, Zaleshoff, and his sister, Tamara. Ambler does so at a cost, however. He distorts history and current events to make the Russians friendly, heroic, ethical, and altruistic. In the process, he trivialized the crimes of Stalin, endorses the Moscow Show Trials, and contributes to the creation of the Trotsky bogeyman. Eventually, Ambler came to recant of these views and his novels became far less political.
But in Background, in the world of 1937, it's enough that Zaleshoff is anti-Nazi. That is what matters. It must have been a shock to the filmgoing audiences, then, when in just a few years, kindly Uncle Joe would sign up to a non-aggression pact with Hitler, invade eastern Poland, and enable the start of World War II. As with many who had succumbed to the lure of the Soviet dictator in order to fight fascism, the filmgoers and writers such as Ambler would soon have their illusions shattered. That is why, I suppose, that so many of Ambler's later books treat corruption, despotism, and widespread massacre as an integral part of human society everywhere.