This question is inspired by a thread in /r/badhistory, where a lot of people were trying to argue that the draft was a form of "slavery" or "slave labor."
Can someone explain the history of the notion that citizens have an "obligation" to fight for the state? When did this notion first become broadly accepted, and what was the political/philosophical justification made for it at the time? Once it had become generally accepted, was there/has there ever been a state that denied or didn't have this requirement (in "western" society)?
Please note that I'm not asking for people to debate whether or not the obligation is "right" - I just want to know about the history of the concept.
Oh! This is a great question! First off, you're right to make a distinction between "the draft" and conscription. They're two very different things!
A Draft is when the Army needs a specific number of men to sustain it's forces. Say it needs one hundred men per month. Everyone's name goes on a big list, everyone is randomly assigned into 100 men groups, and get a number. Basically if your name is pulled out of a hat you and your group have to go, but everyone else is off scott free! (Until the next draft).
Conscription is literally the government goes to town and grabs everyone like a greedy kid at a birthday party's snack table. Instead of a lottery system with some people going and some people staying, during conscription everyone registers and is deployed (during "total" war this 'deployment' might be the government keeping you a civilian, esp if you've got an important job like mining or stevedoring. These are not exempt jobs, they are protected jobs). Don't get me wrong though - with conscription, the government might call up everyone and then even the non-protected folks might be rationed out to the Army slowly, over time. There is still the issue of sustainment to worry about, after all! Of you sent everyone at once, you would suddenly have a massive army you couldn't supply, which would slowly get smaller and less effective over time because you would have no reinforcements to send to cover casualties.
Okay, now we've got that down, let's talk about where these ideas (in their modern form) come from. So we've got to talk about the French.
Firstly, the French are the most war winning people on Earth. Statistically they've got the best win:loss ratio of any nation. They are ball-shatteringly good at war; it is historically their jam. But when they gave their Monarchy the big middle finger for the first time, they ran into a problem. Every other Monarchy in Europe decided that a polity without a Monarch was setting a bad precedent for their own power structures, and invaded. The first war alone pitted France against a coalition of the Holy Roman Empire, Prussia, Great Britain/UK, Russia, French Monarchists, Spain, Portugal, Sardinia, Naples, the Ottoman Empire, the Dutch Republic, the USA, Haiti (technically), and the Knights of St John of Malta.
You can see the French had a problem. But happily, their new ideas about equality, fraternity, and citizenship provided them with a ideological and pragmatic solution. If all men were now citizens and formed the ultimate source from where the State gained authority, then every citizen had the duty to protect the State which they created and which defended them in turn. And so the French state (in the form of the Convention) issued a declaration for something called Levée en masse.
"From this moment until such time as its enemies shall have been driven from the soil of the Republic, all Frenchmen are in permanent requisition for the services of the armies. The young men shall fight; the married men shall forge arms and transport provisions; the women shall make tents and clothes and shall serve in the hospitals; the children shall turn linen into lint; the old men shall betake themselves to the public squares in order to arouse the courage of the warriors and preach hatred of kings and the unity of the Republic"
This is nothing less than the involuntary mobilisation of every single man, woman, and child in France - justified as necessary for the defence of that State and thus the people which constitute it. Although prior to this people were frequently nabbed for military service involuntarily, it was mostly ad hoc and had no justification apart from simple things like "LOL my sword is bigger", "I am your King", or "you can fight or you can get slaughtered by some moors". Levée en masse is totally revolutionary in that it connected involuntary service to citizenship and the defence of the State - it the source of seeing service as an "obligation". (French men still do - or did until very recently - at least one day of training in their lives as a part of this).
Needless to say, being able to put enormous numbers of men into the field saved the French Republic. The first call put 1,500,000 men into the French uniform, and against what seemed to be overwhelming odds. After demobilising them at the end of the War of the First Coalition, the French were then able to call them all back to win the War of the Second Coalition, basically securing a win for both phases of the French Revolutionary Wars.
It was such a powerful tool for winning wars that modern nation-states in Europe eventually regularised and continued the requirement even into peacetime, in different forms. France required a minimum amount of training in amounts depending on your "classes" (what year you were born), for instance, as did Germany. All of these things were strongly ideologically linked to the defence of the polity. Considering the enormous armed camp Europe had become, this justification rang true; at a government level it was considered necessary and wise policy.
One of the sole European exceptions during this modern period was Britain, which due to geographic and social factors, retained a small and highly professional army in preference. They fought colonial wars, and relied heavily on their Navy. Considering the British system almost instantly broke down in the mud and blood of the First World War, this system was replaced in 1916 by conscription (even though volunteers had been plentiful in the early stages of the war). One interesting note about conscription at this time (late 19th century) is it brought a lot of the middle class into the military, increasing literacy dramatically.
"Drafts" are a little harder to pin down ideologically - often it is argued that it is more "fair" and leaves enough men behind to maintain the economy during a 'limited war'. Russia pre-WW1 had a system, for example, where a peasant was simply drafted from a village for 25 years. Essentially the village was given a certain number of men the Army needed, and was told to see to it. Since this was effectively a life sentence the village elders often picked the unlucky victim from the ranks of the mentally feeble or troublesome, which produced soldiers which were less than ideal. The Americans at Vietnam are another famous example of drafting for 'limited' war.
Anyway, I hope this was useful!
TL:DR Conscription came about ideologically during the French Revolutionary Wars.