While the ultimate outcome of The Great War was not decided in it's first month, the nature of the contest was as Barbara Tuchman so masterfully illustrates in "The Guns of August".
From the outset Tuchman shows that all the belligerents made crucial mistakes that slowly mounted resulting the Allied victory at the Marne then to total stalemate for four bloody years. The first 30 days of combat on the Western Front when the entire continent and possibly the world thought it would be a short war, after over 40 years of continental peace, changed everything and everyone it touched along with those it didn't.
In almost 450 pages of text, Tuchman gives an overview of how the war plans that both sides would use in that first month were developed and then showed the history of what happened when they were applied. She filled each page with dense material but with no frivolous words to get in the way. Although in a few places she must, along with the reader guess at what a particular individual commander was thinking at a particular moment she supports her conclusion with the overall situation he faced at the time. Tuchman quoted individuals in their native tongue, however for some one like myself who didn't now any French or German it meant nothing and I had to figure out what was implied by what Tuchman wrote before and afterwards. If leaving unexplained a quote in foreign language is the worst critique I can assess a book, then I'm literally grabbing at straws.
"The Guns of August" was an instant classic upon publication and for any student of history it is a must read. With the 100-year anniversary of The Great War's beginning fast approaching, now is an excellent time to do so.