Are the soldiers there liars or are trendy commentators defending the establishment liars?

The historians believe the words of the soldiers.
Words they left in tens of thousands of letters, journals and personal documents.
Words they wrote at the time.

Overwhelmingly, they knew and understood what they were fighting for, and they believed it the right thing to do.

The "establishment" of 1914-18 are long dead and no-one feels the need to defend them, trendy or otherwise.