Dear Chris, this is a fascinating and important topic upon which far too little has been written.
You will immediately encounter two serious challenges:
1. Scoping the problem. This is an enormous topic and you will have to address questions (for your purposes) such as where "military" intelligence stops and "diplomatic/political/whatever" intelligence begins. What is "strategic" intelligence as compared to "tactical," etc? No inherently obvious answers, so you will likely have to select a course and than defend your definitions.
This is all surmountable, but recommend you sort this out for yourself before approaching your thesis adviser.
You are probably best refining your topic to a specific country (how did French or Prussian or Andorran military intelligence evolve over time, if it did?) or to a specific conflict (probably not enough material to work on a specific battle).
2. Sources. There is very little in the secondary literature--at least very little that will support the type of in-depth analysis you doubtless want to undertake. What you will find in published material is often at best every tenth or twentieth dot in the connect-the-dots exercise. There is material in various archives, but it can take some searching. Moreover, by its very nature, intelligence is murky (letters not signed or addressed, etc.) and there is much that was not recorded or was soon destroyed or was only conducted verbally. Suspect you will have to work hard to craft a useful thesis topic if you do not have access to an archive. Not impossible, but will take some careful thought.
There will be other obstacles too, no doubt, but those are two initial ones to consider as you decide whether to grapple with this important, but complex and elusive topic.