Named for the most important late Roman work on military affairs, written around 390 by Vegetius, this is the major organization of medieval military historians in the United States.  While the majority of members are from this country, a substantial minority comes from Canada and Great Britain.  

ARMA:  The Association for Renaissance Martial Arts:  http://www.thearma.org/

You want to become a medieval warrior?  This is the place to do it!  ARMA, an organization now headquartered in Atlanta, teaches people medieval and Renaissance fighting techniques with a variety of weapons, based on a careful examination of the surviving literary and pictorial sources from the period.  The Association, that combines scholarship and hands-on experimentation, is headed by John Clements, America's foremost medieval-Renaissance martial artist. Members of the Association are not simply reenactors.

TemplarHistory.Com:  http://www.templarhistory.com/

Although this is a commercial site and suffers from a superabundance of advertising, it is the best Templar site among many on the web.  (There is currently some indication that the advertising arm of the order may soon be split off to a separate site.)  For the most useful material, including a collection of documents dealing with the Templars, click through to the History section.

Tales from Froissart:  http://www.nipissingu.ca/department/history/muhlberger/froissart/tales.htm

Born in Hainault, a duchy in the Low Countries, Jean Froissart was the most important chronicler of the later Middle Ages.  In a life of travel, he encountered many of the people whose stories he would record, writing and rewriting a sweeping chronicle of late fourteenth century Europe, much of it devoted to warfare.  Steve Muhlberger of Nipissing University has made a number of sections of the chronicle available on the web, using for the purpose the standard nineteenth century, two volume translation by Thomas Johnes.

NetSERF:  The Internet Connection for Medieval Resources:  http://www.netserf.org/

A play on words, Netserf contains a user-friendly bibliography of medieval sources by subject.  Included are entries on Military History, Feudalism, and Heraldry and Genealogy, all of which have a bearing on medieval warfare.

The Medieval Sourcebook:  http://www.fordham.edu/halsall/sbook.html

This source is posted by Fordham University, one of America's leading centers for Medieval Studies.  It is part of Fordham's on-going Internet History Sourcebooks Project (http://www.fordham.edu/halsall/ ), begun in 1996 and continuing into the present.  While the sourcebook is not exclusively or even primarily military, it contains many medieval documents which will interest scholars studying war in the Middle Ages.

The English Calendar of Patent Rolls:  http://www.uiowa.edu/~acadtech/patentrolls/

The printed Calendar of Patent Rolls, consisting of 49 volumes and covering the years 1216-1452,  has long been a major source for medievalists working in English history.  It contains a record of patents issued by the crown conferring all sorts of royal grants conferring property and titles, pardoning crime, protecting individuals in the performance of their duties or business, etc.  Quite a number of these patents shed light on military affairs in England.