The institution of Army has evolved smoothly and naturally from the feudal times, when the lords were obliged to deliver to the king a certain number of soldiers (knights, men-at-arms and yeomanry) to fight for him upon request.
The number of soldiers depended on the size of land the lord got from the king, and the number of people populating it. The people were counted as the lord’s property, along with the Flora and Fauna.
Those accidental bands were forged into the first professional army, called the New Model Army, as a result of the Civil Wars of 1642–1651.
It cost the total of 868,000 combined casualties on the English, Scottish and Irish sides (about 11.5% of the population.)
A commissioned illustration of the Battle of Naseby (1645). Note the lack of blood and the delicate postures.
Even though it was called a Civil war, it was quite savage.
The opposing parties were numerous and hard to distinguish between: Royalists against Parliamentarians; dissenting Nonconformist religious sects; English against Scots and against Irish, Catholics against Protestants, and many shades in between, as allegiances were shifting along with the outcome of various battles and the ever-changing anticipation of who will ultimately win.
War has always had manifold advantages for its initiators: it was - and still is - good for the business of arms-trade, it is a great excuse to raise taxes, a good camouflage for channelling national funds for personal needs of the one doing the channelling, and an excellent way to keep the numbers of the populace low enough for the rulers to control.
When that war was exhausted, in 1651, it drifted to the seemingly cleaner fields of politics, with new opportunities presenting themselves to send young people to kill each other, first in 1648, with the Second English Civil War and then in 1649 with the Third Civil War.
Eventually the Army itself became the target of the conflict between the Parliamentarians and the Monarch. The Bill of Rights, which the English Parliament passed in 1689 defined the new power balance, preventing the Monarch from keeping an army in peacetime, without the consent of Parliament. A tradition that is kept to this day, when Parliament must convene at least once a year, to renew the Army Act, as it is now called.
The rest is a bloody history of wars, almost all over the rest of the world, all legalised in parliaments and blessed by religious authorities.