They were on the march, momentum behind them, and almost certainly believed victory was on the cards.
To obtain doctrinal support, he took advantage of competition within the Church that had been sparked by Martin Luther's Protestant reformation in Germany.
The result was that the religious conflicts in mainland Europe were brought over to Britain, dragging in monarchs from across the Channel as Kings and Queens were moved around in a giant game of ideological chess.
In 1689, prominent British Protestants invited the Dutch 'William of Orange' to invade Britain and ascend to the throne with his wife, Mary II of England.
The object was to depose the Catholic James II of England and Ireland (and VII of Scotland) and ensure a Protestant bloodline held the crown.
(Mary was Protestant, even though her father James was a Catholic).
56 years later, Scottish supporters of James II's grandson, Charles Edward Stuart, or 'Bonnie Prince Charlie', were fighting to get their man back on the throne.
It wasn't the first time they'd fought to reassert Catholic supremacy within Britain's royal house - in 1715 the first Jacobite rising (known as "the Fifteen") had ended in defeat when the first of the Hanoverian kings of Britain, George I, had dispatched his forces to beat them at the Battle of Preston.
(George I, who was from Hanover in what is now Germany, had also been made King of Great Britain because he was Protestant. William and Mary, who'd been first cousins, had died childless, and when Mary's sister Ann, an Anglican, had died after ruling from 1702 to 1714, Catholic relatives of Mary and Ann were bypassed so that the crown could be given to George).
Fort George, in its first incarnation, was constructed after the 1715 rebellion as a bulwark against future Catholic/Jacobite uprisings.
Thirty years later, "the Forty-five" rebellion unleashed the Jacobian revenge. They may have failed to take Stirling Castle or Fort William, but Fort Augustus, and most importantly, the king's namesake, Fort George, they captured.
In 1746 they blew it up.
But the Jacobite train, so far rumbling to some important victories, was about to smash straight into better-armed, better-trained, and better-organised government troops.
Like the Battle of Hastings in 1066, 1746's Battle of Culloden saw two types of warfare ranged against each other.
Unlike Hastings, which took all day, Culloden's carnage was even more one-sided, and victory for the 'redcoats' was delivered within an hour.
A failed Highland charge, mounted by the Jacobites bearing cutlasses, was almost a foregone conclusion. Redcoats, trained to load and fire at least three rounds a minute from their state-of-the-art Brown Bess muskets, took down between 1,500 and 2,000 Jacobites. There were only 50 dead amongst government troops.
Bonnie Prince Charlie gave the order "Every man for himself" and managed to escape the field.
George's forces had won, and military planners soon got to work on devising the next fort to stop future rebellions.
They started by learning from past mistakes. The second incarnation of Fort George would not be within Inverness itself, where it too might one day be captured and destroyed.
Instead, it was sited at Ardersier, a small fishing village 11 miles away.
From this vantage point, based on a prominence jutting out into the Firth of Moray, the new fort could control the sea approach to Inverness as well as be resupplied that way if it was ever besieged on land.
Started in 1748 and completed in 1769, the original cost estimate of £92,673 19s and 1d overran to, what was at the time, a massive £200,000. According to STV, this was more than Scotland's entire GPD in that period.
The fortifications were the most advanced of their kind for the period, utilising a strategy of defence in depth that incorporated a patchwork of jutting redoubts and bastions enclosed within massive outer walls.
Cannons were set up and positions pre-sighted to ensure that any attack on any wall was within the arch of fire of at least one gun.
Loose shingle on the ground outside the castle would have denied an enemy any chance to bring up his own heavy guns.
What's more, a bridged entrance within the fort's outer walls, spanning a drop that could be turned into a moat by pulling up locks and letting in sea water, was also open to musket fire from all sides, making an infantry attack impossible too.
Perhaps, though, the fort was a victim of its own success. With the Jacobite threat so effectively put down at Culloden, Fort George may have already become superfluous. Coast's Neil Oliver has described it as having become a white elephant.
It has certainly never been more than a barracks, first for the Seaforth Highlanders (formerly the 72nd and 78th 'Highlanders' Regiments of Foot) from 1881, then for the Queen's Own Highlanders (amalgamated from the Seaforths and Cameron Highlanders).
In 2007, 3rd Battalion, the Black Watch became garrisoned there.
Now, according to STV, its future as a barracks is uncertain. Along with Kinloss Barracks, Fort George is apparently being considered for closure because of the latest round of spending cuts.
While the Jacobites would have been delighted by such a turn of events, today's Scottish government is firmly opposed to such cuts and fiercely guards what is now home to 3 Black Watch.
The Jacobite cause never regained momentum after 1746. The French tried to entice Bonnie Prince Charlie (Charles Stuart) to ally with them during the Seven Years (or French and Indian) War of 1756 to 1763.
But by that time, many of the Scottish Highland forces that had opposed the Hanoverian government had now been incorporated into the Army and fought nobly for the British against the French in that war.
And for all his youthful vigour, 'Bonnie Prince Charlie' was now a shadow of his former self. According to the BBC's Simon Schama:
"Unhappily for the keepers of the Jacobite flame, Charles Edward in exile went rapidly downhill. Too many mistresses, far too much drink, years of indolence made him prematurely decrepit."
The House of Hanover, meanwhile, went on to rule Britain until the present day, or rather its bloodline did.
Queen Victoria was the granddaughter of George III, and, as the niece of his son William IV, succeeded him to the throne in 1837.
She married Prince Albert of Saxe-Coburg and Gotha and so on her death in 1901, her eldest son, Edward VII, became the first British monarch of today's royal house, that of Saxe-Coburg and Gotha.
Or so it would have been, were it not for Queen Victoria's grandson William, or Wilhelm, the King of Prussia and Emporer of the newly unified Germany.
Bombastic and insecure about his withered left arm, Wilhelm II, the son of Queen Victoria's eldest daughter, was a major destabilising force on the continent.
As a young man in the German Army, Wilhelm's insecurities soon began to influence him there.
According to 'The Great War and the Shaping of the 20th Century':
"Wilhelm's wardrobe contained some 200 uniforms. He delighted in wargames, but demanded that his side always win. Queen Victoria now found her grandson insufferable, and suggested that he would benefit from a good spanking."
As it turned out, it would be the British Army (along with the French and the Americans) that would give him the spanking, but not before anti-German sentiment in Britain had also affected the British monarchy.
In 1917, Saxe-Coburg and Gotha was quietly dropped, and 'Windsor' taken in its place.
With the end of World War I in 1918, Kaiser Wilhelm was forced to abdicate and went into exile in the Netherlands where he died in 1941, having been the last emperor of Germany.
To find out more about the Jacobite uprising, read 'The Scottish Jacobite Army 1745 - 1746' by Stuart Reid and 'Culloden Moor 1746', also by Stuart Reid. Visit Osprey Publishing for more military history.