I read this at a couple of sittings and enjoyed every bit of it. It's like Marxism; read this and you have the answer to everything. The ideas underpinning in it are strikingly simple, though only one of them is fully articulated.
First, even now countries are limited or enabled by their basic geography - where they are on the globe, whether they have rivers,mountains, natural harbours, fertile soil, forests or deserts. Marshall makes out a compelling case. Second, geopolitics - geographically influenced or determined political possibilities and necessities - geopolitics is always Realpolitik. Your neighbours are unlikely to be your friends and you have to prepare for the worst. It's always going to be Them or Us, a zero-sum game. You must always be ready to fight.
It's this second, less articulated theory which gives the book its Boys Own Annual feel. They ( usually China, Russia) are out to get us and they will get us if we don't get there first. It's true that Marshall quite often shows understanding for and even some sympathy for what they are about - he gives a very good account of why President Putin felt he had no option but to take Crimea - but his suspects are the usual suspects and he has a Foreword by Sir John Scarlett (Tony Blair's man at MI6 and not exactly a persuasive choice) to back him up.
Keep that reservation in mind but do read the book. It's very well done and full of ideas and small asides you will never even have thought of. He does, for example, give an interesting geographical explanation of why the Americans eventually decided to use the H-bomb on Japan though it doesn't explain why they didn't give the Japanese a demonstration of its power on empty land before they used it on real cities. True, that would not have had the shocking power of the real thing and the Americans felt that it was the viciousness of the Japanese military spirit - don't forget their war crimes - which had to be broken.