this is a nice book written by a former pakistani general, but i find that some of the information in there seems to be based on legend as much as it is fact, still a good read though.

As to what made him an effective leader, you have to look into the environment he was born in, specifically his tribe. So Khalid was born of Quraish, who were the rulers of Mecca, a relatively wealthy town in arabia. his tribe was split into a handful of clans that split up the tasks of ruling Mecca. His clan,Banu Makhzum, were responsible for matters of war, and they were also practically the wealthiest of Quraish's clans.

So Khalid, being the son of the tribe's war-chief, grew up with the expectation that he would take on that role, and being wealthy, he also had the time and resources to devote towards learning the martial-arts and traditions. Everything from wrestling to swordsmanship, horsemanship to tactics and strategies, Khalid steeped himself in training and learning, and from his words (and accomplishments) it seems that he relished battle at least as much as he felt the social obligation to engage in it.

Another thing to consider was the nature of the environment. Arabia is an area that had long been characterized by endemic-warfare, so much so that they had rules as to what particular months in the year and what area's that tribes can raid each other, what constitutes spoils, what obligates an act of vengeance...etc. So its safe to say that Khaled had an arena in which to apply his training and knowledge, and to gain valuable experience.

Moving on to his glory-days; the man had achieved victory after victory even before the coming of islam, and once he joined mohammad's cause, he had access to a battle-hardened, incredibly zealous and highly-motivated force, the core-group of which already respected his leadership skills and his battle-field prowess, his intelligence, generosity and fairness, especially in the dispensing of booty. And of course, there was that not-so-insignificant fact that he hadn't lost a battle...ever, and who doesn't like to roll with a winner?

This reputation would always proceed him, drawing more soldiers to his ranks, soldiers who were more than happy to serve under him, which raised morale even higher, which, much like Ceasar Augustus and Alexander, allowed him to push his troops harder than most commanders could, to the point where his troops would fight heavily outnumbered for six days in the [battle of yarmouk](http://en.wikipedia.org/wiki/Battle_of_Yarmouk} without a hint of sedition.

of course there is more to be said in terms of the circumstances, the waning state of his enemies (romans and persian), and the state of his troops and how he organized, motivated, and disciplined them, how he utilized the particular tactics that Arabs of the time excelled at, and how he took advantage of the weaknesses inherent in both his enemies that constrained them... etc. but i think i'll leave something for a true historian to add.