I’ve been thinking about the strange claim among some circles that the American colonists in 1775 were plucky underdogs who went toe-to-toe with the world’s most powerful empire and defeated it. While it’s true that Britain probably was the world’s foremost power in 1775, it was only so because the Spanish were approaching the tail-end of their own abilities and France was in its post-Seven Years War slump. But even disregarding the fact that revolutionary victory was dependent on the world’s second and third most powerful empires backing the colonists up against the first, it’s really a huge reach to describe the Whigs as underdogs.
The first reason for this is that the British Army plus German allies totaled 194,000 men at its height during the war, though 42,000 of those were militiamen relegated to Britain. That left 152,000 soldiers to not only defend a global empire against attacks from two other global empires (three if we count the lukewarm Dutch support for the American revolutionaries) but also suppress a rebellion among a population of well over 2,000,000 people (approaching 3,000,000 by 1780). It is estimated that of this total 6.5% of the population were actively involved in the revolutionary struggle against Britain - over 130,000 men. In its most simplistic terms, the revolutionaries in the Thirteen Colonies were able to muster almost as many soldiers as the entirety of the British empire, and that’s before said imperial forces were divided across the globe or kept in Britain to defend against a French invasion. This is also without factoring in the tens of thousands of French and Spanish forces either supplied to assist the revolutionaries directly or deployed separately against other British holdings.
The upshot of all this is that Crown forces were outnumbered in almost every single battle and skirmish of the revolution. Britain was only able to deploy, at the height of the war, around 70,000 soldiers to North America without overextending itself elsewhere, and even with the support of Loyalists (who were far less active militarily than the revolutionaries) they would still be outnumbered at a ratio of nearly 2:1, even before the inclusion of French and Spanish forces. This disparity really comes out in the sources - British commanders like Cornwallis were continually aware of their parlous state, frequently stranded in hostile territory, surrounded and heavily outnumbered. That sort of situation, repeated as it was so frequently, is not the natural state of a superior fighting force.
The maths provides the first reason why the Whigs weren’t the underdogs. The second comes from the nature of the fighting men themselves. The British Army in 1775 was no well-oiled machine. The last major conflict it had been involved in had ended 12 years earlier, and the intervening period had seen the government all-but purge the military in a series of huge cost-cutting exercises that reduced it to barely 20,000 men. The only soldiers with experience were a scattering of officers and NCOs. Almost all the regulars had never seen action before 1775. Ironically, the colonists had more military experience due to the intimate nature of the Seven Years War in North America. Many militia officers had served against the French and Indians. It has been estimated that at Lexington and Concord, the militia - the supposed underdogs - had more military expertise than the green regulars sent to face them. Both sides learned to fight as the war progressed, and while the pre-existing structures of the British Army gave it a certain advantage over the more amateur Continentals, man-for-man there was very little separating both sides. The British were certainly not a highly experienced and immaculately drilled fighting force.
tl;dr Britain wasn’t an exceptionally powerful empire and the American colonists were more numerous and experienced than popular history allows. Also France.