Sentence:  [O]ne minute this "Jihadi John" was struggling to get by, and get accepted, in drizzly England, unemployed with a mortgage to pay and a chip on his shoulder, and the next he stands in brilliant Levantine sunlight, where everything is clear and etched, at the <WSD>vanguard</WSD> of some Sunni Risorgimento intent on subjecting the world to its murderous brand of Wahhabi Islam. 
Meaning of the word marked with <WSD> tags:  The person(s) at the forefront of any group or movement.