That army was hidden within the woods of Abbey Craig, a hill overlooking the river Forth spanned by Stirling Bridge.
Wallace, a nobleman who'd refused a call to service by England's King Edward I, had chosen to help whip up a rebellion instead, and joined forces with Andrew Murray (or Moray), another nobleman who'd fought, been captured by, and then escaped from the English.
The two men had around 180 cavalrymen and 6,400 infantrymen, including bowmen for protection, under their command.
These soldiers were every bit as well organised as the English forces that would soon deploy in front on them on the morning of September 11th, 1297.
The infantry was organised into 'schiltrons', a body of troops formed into what was effectively a phalanx, armed with pikes or spears.
A quaternion had four men under his command, making up, with him, the equivalent of a five-man squad. Two of these units were commanded by a decurion, which was roughly the equivalent of a 'section' in today's British Army.
Men were further designated into company-sized units of 100 men, and regimental groups of 1,000 men were commanded by a chiliarch.
For their part, the English forces were over-confident and disorganised.
A full-blown war between England and Scotland had begun the year before, but Edward I, convinced he'd crushed the Scots, had left.
Muttering 'it's a relief to be rid of s**t', he headed south again to deal with more pressing matters related to the crusades and his lands in Southern France.
John de Warene, the Earl of Surrey, and Sir Hugh Cressingham, the Treasurer of Scotland, were the king's representatives left behind at Stirling.
The town had a castle cresting a hill that was opposite the Scottish rebels, who were now encamped inside the woods on Abbey Craig.
Stirling was strategically vital - the River Forth separated northern and southern Scotland, and any invasion of the north would need to start here.
In his book on Wallace's rebellion, however, author Pete Armstrong suggests that Cressingham may not have been militarily astute enough to fully appreciate this.
As treasurer, Cressingham had a lot more troops under his command, but he'd sent a large number away, deeming them unnecessary and too expensive. This left him with 350 cavalry and 6,350 infantry (including long and crossbowmen).
On the morning of the battle, he'd rejected advice to send a party to ford the river further north so that his men could flank the Scots, who were now visibly assembling opposite, below Abbey Craig.
He may have been concerned that such a manoeuvre would cause the Scots to flee, and planned for a quick victory over the rebels that would curry favour with the king and advance his career.
So he chose to have all his men cross the narrow bridge and assemble into battle lines on the other side, not realising the danger posed by dividing his forces.
The Scots, using their opponents' complacency to their advantage, awaited the perfect moment to catch them off guard.
It was a fight that had been a long time coming.
Braveheart opens with a dark prologue - Scotland, with a recently dead king, is vulnerable to England's cruel and opportunistic 'Edward the Longshanks'.
The dead king mentioned was Alexander III, who'd died not in 1280, as the film states, but in 1286, when he was flung from his horse and plunged over a cliff.
His death triggered a political crisis because there was no clear heir, and Scotland's nobles, fearing civil war, asked Edward I to arbitrate between them.
Over time, the process became less about helping Scotland find a king and more about helping Edward find a loyal deputy.
By 1292 Edward was ready to give his approval to a man he thought he could trust to be his political servant, and John Balliol was crowned king of Scotland.
But in the months and years that followed, relations soon soured between them.
Edward was perceived to have interfered with Scottish sovereignty when he intervened in a judicial matter. Balliol was perceived as traitorous when he refused to support Edward in a war with France, and then signed a defence pact with the French king.
War broke out between England and Scotland in 1296, and Balliol was eventually captured and imprisoned in the Tower of London.
The whereabouts of Wallace at this time are unknown, but, given the leadership skills he displayed at Stirling, it is very likely he'd had prior military experience.
He may have fought with Edward's forces in Wales or on the continent, and probably against them in the war of 1296.
Now, with the English king missing in action, and his delegates clumsily filing over the bridge ahead, it was the perfect moment to strike.
With a third of the English forces now over the Bridge and cut off from reinforcements, Murray and Wallace gave the order to rush them.
In an instant, more than 6,000 Scots swept down from the base of Abbey Craig and slammed into their exposed enemies.
The schiltrons formed up, no doubt bristling like porcupines, six lines deep, with the better-armoured men at the front.
Wallace and Murray likely swept down on horseback along with the rest of the Scottish cavalry, hacking at any enemy soldiers unlucky enough to be caught in the open.
Across the bridge, the remaining English forces helplessly watched as the Scots made quick work of slaughtering their comrades and forcing them into the river, where most, encumbered by heavy weapons, clothing, and armour, were drowned.
After Stirling, Edward was forced to re-engage with the war in Scotland and Wallace suffered defeat at the Battle of Falkirk in 1298. Murray, meanwhile, had died from injuries sustained at Stirling.
Wallace was captured in 1305 and put through a grisly execution at the Tower of London, his body 'quartered' and limbs dispatched for display in Newcastle, Berwick, Perth, and Stirling.
So the Scots were down - but they weren't out.
In 1314, another icon of Scottish history, Robert the Bruce, emerged victorious at the Battle of Bannockburn, his feat of splitting an English knight's head in two with his axe becoming as legendary as Wallace's victory at Stirling.
The Treaty of Edinburgh-Northampton subsequently acknowledged an independent Scotland and its king, Robert I, as ruler.
For more about Stirling Bridge and Falkirk, see Pete Armstrong's book. For more about the Battle of Bannockburn, click here. And to learn more about military history, visit Osprey Publishing's website.