A historian will study contemporary documents and source material, and base his findings on verifiable facts.
His rivals will then scrutinise his work and expose any inadequacies or false conclusions.
Very much like Science.

As with Science there are often differences on some issues, but a consensus on others.

There is a consensus among historians about aspects of WW1.
I base my views on that.

Musket has been unable to find one historian to support his views.

Like a fundamentalist, that just makes him more adamant.