The year 1642 produced an outbreak of hostilities which later took the shape of a civil war. That civil war was not a single conflict, but a chain of conflicts which changed the country for all future generations.
The original and direct combat was between Parliament and the royal forces of King Charles I. Both opposite sides saw the necessity for shaping England’s own army to withstand such foreign threats as the Spanish Armada, North-African pirates, the powerful and aggressive Dutch fleet etc. But King Charles I was unable to meet all challenges coming from the reality, as well as the demands from Parliament.
Not only the nobility, but ordinary people were involved in the military actions. The country was split mainly in geography. There was almost no social division, because tenants and servants supported mainly the same war opponent as their gentry did.
Different parts of England were loyal to the different sides of the war. But London was predominantly supporting Parliament. The eastern territories also adhered to the parliamentarian policies, whereas Cornwall was significantly royalist.
In the beginning of the Civil War, England did not have a capable army. Its population was not used to weapons or military tactics. In the outcome, the army became modernized and sophisticated, the common people got accustomed to defending their homes and land. The country became capable to withstand any foreign menace.
In the city of York, the royal subdivisions were entrapped and had to fight with the Parliamentarian and Scottish squadrons in 1644. The York battle became one of the bloodiest fights in England. The royal army was defeated in it.
That short-term military evolution led to an entire change in all the future history of the state. The modern British Army roots back to that very historical evolutionary period.