I didn't see a rule that directly applied to this question so feel free to remove it if it doesn't comply.
Yes, I realize the question is broad, but I'm looking for books that can help me narrow that down as I read. For example, I remember reading or hearing once that there was no adequate way the Alamo could have been defended. I have always wondered how and who made that assessment. Looking back, it was probably a matter of people power and firepower, but my question is, what are some engaging, accurate, and, most importantly, accessible places to learn the history and evolution of military strategy? Not in a, "I need to learn how to outflank my opponents" but in a general, "why did Lee do x during the Battle of Gettysburg?"
If necessary, I'm most curious about US military strategy because I feel like I have the most context for it. That being said, I'd love to hear about any others as well.
Military Theory is one thing Generals love to write about, or at least they used to.
For broad strokes two of the best writers to start with are Jomini and Clausewitz. Both of who wrote in the aftermath of the Napoleonic wars and attempted to make sense of the massive shift in military dynamics and war which had occurred within a generation. Was warfare about applying force to important enemy points, or was it about destroying the will and ability to resist? Was strategy something that could be quantified, or could a leader simply rely on training and respond as a situation developed?
Jomini in particular was VERY widely read at West Point, and would have been familiar with virtually every professional officer during the Civil War.
On the Naval side of things I would recommend, Alfred Thayer Mahan. His works during the end of the 1800's essentially dictated naval thinking in large part even to today. He also did much to improve the standing of the USN in the eyes of the world, during a time of rapid expansion and the quick victory over Spain. Essentially he demonstrated in his work (The Influence of Seapower Upon History) that the nation that controlled the sea had an inherent advantage over anyone else. And that the best way to seize it was to destroy the enemy fleet, and argued that fleets should ebb kept concentrated in order to both protect themselves and to brig the enemy to battle when the time was right.
None of them are going to be shallow or brief reads, but taking them right from the source can help inform any later reading and help you understand the thinking of military leaders.
I'd like to recommend Patterns of Conflict by John Boyd. You can look up the wikipedia article on him here. Basically, he's at least partially responsible for why some advanced US weapons systems (the F-15 and F-16 for example) are designed the way they are. He's also thought to have had a great deal of influence over US military strategy in the 1991 Persian Gulf War, far and away the most successful military conflict the US has been engaged in since the end of the Second World War.
Keep in mind, Patterns of Conflict was a lecture, not a book. You can find Boyd's old lecture notes if you google the title. They're certainly worth reading. The concept of OODA loops is something worth exploring as well.
I liked Azar Gat's A History of Military Thought: From the Enlightenment to the Cold War (Oxford 2002). It is an overview of war, strategy and military thought. It covers all major military thinkers and places them in context, so you'll be able to see the 'why' very well.