I read this book about 20 years ago, so my memories of it are not fresh, but I remember finding it a very engaging work that does a good job of sweeping the reader up into the drama of the first weeks of the war. You’re correct, though, in noting that Tuchman quite freely imputes motives for people’s actions that can’t be substantiated. As I recall, Arthur M. Schlesinger, Jr., did this a lot, too. I also seem to remember that Tuchman pretty much follows the “blame Germany” interpretation for the war that (in my opinion) is not really tenable.