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French surgeon Ambroise Paré (c.1510–1590) is considered one of the fathers of surgery; he was leader in surgical techniques and battlefield medicine, especially the treatment of wounds.
This sentence is from Wikipedia's scientific revolution article.
Why we don't say "he was a leader" or "he was the leader", is "he was leader" right?
In the given context you should say "he was a leader."
If it said he was leader of the battlefield surgery and wound care squad that would be perfectly acceptable. But, it is speaking of an abstract in the form of a technical field of study.
The other form you've asked about "He was the leader", has a different meaning in context. This would mean that of all the surgeons he was the single best in this field who came up with all of the major advances on his own.
Compare this to "he was a leader". Which means that amongst the surgeons he was one of the best in the field. One of the men responsible for the major advances in the field (but, there could have been others).
As you see, shifting the article from a to the has a dramatic shift in meaning.
In the context, it's not right. "He was a leader" would be the right form, since there were many pioneers working in the same field. Where "he was leader" might be used is when referring to a small group of people engaged on a task, where different roles have been assigned. "He's leader, Tim is lookout, Charlie is point-man, Fred is general back-up..." etc.
On its own there is nothing wrong with "he was leader", however this would mean that he was the leader of a group: "he was leader of parliament", "of these the warlike Polypoetes was leader" etc. However this meaning of leader indicates that he was the commander/captain/chief. This usage has an implied "the" and is often used with such titles as "he was king/lord/duke etc of somewhere"
However, "leader" when used as a synonym for "pioneer" usually implies that there were others working in the same field or that others have come after his time, thus this should be "a leader".
"He was leader" is ungrammatical; it sounds like verbatim translation from another language (first guess would be French, of course). You would normally say "He was a leader", but even then, in this situation, it seems like the wrong word. "He was a pioneer..." is probably better word choice.
No "he was leader" is (borderline) ungrammatical. You can say "he was a leader" if he was one of many early surgeons, or "he was the leader" if he was by far the most important early surgeon.
