'Murica - Happy 4th of July

"13 COLONIES IS KIL"
"FIDDLESTICKS"

France and Spain were both openly at war with Britain, and other powers (e.g., the Netherlands and Prussia) were aiding the US with money and military expertise. Gibraltar was under siege. The Spanish and French navies were capturing British resupply ships on the way to North America. The war was adding millions of pounds of war debt on top of the existing debt from the Seven Years' War (which was already difficult to pay when they could no longer rely on colonial taxation).

So while it's possible to say Britain certainly had the military might to win the war if they wanted, that's like saying the US had the military to win Vietnam if they wanted--with total disregard for costs, fatalities, or geopolitical consequences.