Thomas's book is often the first people read when they come across the SCW (Spanish Civil War). It is a monument to a lifetime of work and revision (being updated four times for this present edition) and a pioneering work of scholarship. However, it was written in 1961 and although it has been updated to try and include new waves of research, it is woefully inadequate and these shortcomings become more apparent in new books on the subject. This is not a reflection on Thomas, but on the time he published the book.
The work itself is generally written from a high-political position, that is he deals with the decisions made by generals and politicians rather than on the ground. This allows the reader to engage with all the issues as they unfold; from military decisions of Miaja and Rojo to the political intrigue of Franco and the various elements of his coalition. It is also an incredibly detailed narrative history, placing the reader at the heart of the moment with an incredibly well-written chronology. This, I feel is the book's greatest strength. This is hindered by its ENORMOUS length but for those wanting a one-volume work frequently turn to Thomas as a reliable and "neutral" observer. This is where my objection sets in.
Thomas himself is a notorious conservative (small c) and frequently reflects a centre-liberal position. This is not a major issue as he still incorporates evidence from the left and right. However, the way in which he does this is a real problem. If you take a look in his bibliography and footnotes you will find many accounts of atrocities or military escapades are supported by what are now accepted as Francoist propaganda. Particularly, Herbert Rutledge Southworth's work on the many lies and myths produced and developed by the regime and Preston's commentary on right-wing historiography undermine many of the claims made by Thomas. This is not entirely his own fault as he was writing in the late 1950s when the range of material was quite narrow but recent critical works have shown many of them to have embellished - if not invented - evidence. It just means that to the extent the book is heralded as "neutral" and "objective" should be treated with a severe health warning.
One example (mentioned by another reviewer) is that of Ronda and the massacre of 500 from the top of a cliff. Investigations by Buckley and Corbin (made more difficult by the Franco regime's mass destruction of documentation) have shown that this is a total fabrication and that the scale of the killings was much less, not committed at the location and frequently committed by PERSONAL not POLITICAL conviction. This is on instance. The work is peppered with accounts which are taken at face value and should be - and have been - genuinely challenged. Radcliff's work on the "anarchist" stronghold of Gijon has produced a different outcome and native Spanish historians - Moreno Gomez, Casanova, Espinosa among others - have highlighted the incredibly suppressed but brutal and destructive importance of Nationalist violence. Richards' book "A Time of Silence" really gets to grip with this.
Overall I cannot overstate the significance of the book as a historical landmark. It provided the gateway to generations of historians on the subject and the work is rightly seen as an excellent and pioneering contribution to the history of such a contested conflict. However, I have serious reservations about the age of the book despite numerous updates. It is a monumentally detailed history but unquestioningly incorporates Francoist and Rightist propaganda whilst not taking account of the post-war feuds within and between leftist factions.
It will continue to be a testament to the conflict but one should read it with caution. I strongly recommend Graham's book on the Republic for a more academic assessment or Beevor's popular history which is accessible and takes account of the plight of the vanquished.