Definition of "vanguard"

They ſay, that the King diuided his Armie into three Battailes; whereof the Vant-guard onely well ſtrengthened with wings, came to fight.

1622, Francis, Lord Verulam, Viscount St. Alban [i.e. Francis Bacon], The Historie of the Raigne of King Henry the Seventh, […], London: […] W[illiam] Stansby for Matthew Lownes, and William Barret, page 35

By some paradoxical evolution rancour and intolerance have been established in the vanguard of primitive Christianity. Mrs. Spoker, in common with many of the stricter disciples of righteousness, was as inclement in demeanour as she was cadaverous in aspect.

1922, Ben Travers, chapter 4, in A Cuckoo in the Nest‎

[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 vanguard of some Sunni Risorgimento intent on subjecting the world to its murderous brand of Wahhabi Islam.

2014 November 17, Roger Cohen, “The horror! The horror! The trauma of ISIS ”, in The New York Times‎