Up and waur them a':

The tune as "Up to war a' Willie" is in the 2nd book of Walsh's 'Caledonian Country Dances', c 1736, and in several later Scots tunes collections prior to the appearance of Burns' song with it as #188 in 'The Scots Musical Museum', (1788).

Burns' song is a revised and slightly extended version of a seven verse song in 'The Charmer', (2nd ed) 1752 (not seen), where the author's intitals are given as B. G. I've seen it also in the 3rd edition of 'The Charmer', 1765, but I didn't copy it. All I have of it is the first verse as quoted in James Dick's 'The Songs of Robert Burns':

When we went to the field of war
And to the weaponshaw, Willy,
With true design to stand our ground,
and chase our faes awa, Willie;
Lairds and lords came there bedeen,
And vow gin they were pra', Willy,
Up and war 'em a', Willy;
War 'em a', war e'em a', Willy.

I don't think anyone really knows what the original song was. I've found one in a MS dating from shortly after the Battle of Culloden (1746) that starts:

Up and rin your wa' Willie
Up and rin your wa's
Cullodens Lawrells you have lost
That gain'd you such applause
A check of conscience for your crimes
That stings you to the Saul willie
Has broke your measures this Campaign
As much as Lewdendhal's willie
Up and rin &c.

But this, of course, is not about Mar's rebellion of 1715.