Conscription, one of the most dreaded words to the American public, is quite common in the rest of the world, and has been in use in the United States since the introduction of selective service in 1917. While seen as a rite of passage in quite a few areas of the world, it signifies that the United States is at a war severe enough to be seen as a threat to the entire populace itself (from the threat to free trade in 1917-18, democracy in 1940-45, and capitalism in 1948 on). Whether the government was correct in its assessments is meaningless, but it has given a look at who is expected to fight and die for reasons ranging from survival of the country to mere ideological enforcement across the globe against opposing forces.
While conscription is common in Africa, the Near-East, the ex-USSR, and Latin America, it is not a feature of most ‘developed’ countries. What the countries with conscription have in common is that the state’s desire for security (internal or external) is strong enough that a volunteer force from their domestic population would not work, be large enough, or be desirable. From a pure classist perspective, in most ‘developing’/industrializing countries, most of the population of a conscript military is going to be proletarian in origin, due to the changing nature of their country. For some countries, conscription is a valuable tool for giving the lower classes of the country work while simultaneously providing education that is hard to come by otherwise (but both of which could be accomplished through a civil service). Other states use the conscription as a source of cheap cannon-fodder, giving no regard to the individual soldier beyond the fact that a loyalty to the state is instilled and a desire to liberate themselves oppressed.
The United States is a post-industrial country with a large population, a common appreciation for the military and the state in popular culture, which drives a large amount of volunteers, an educational system that provides basic education, a large fraction of civilians who own firearms, and a lack of military foes. This eliminates the need to conscript, but the Selective Service is always present for legal males who are of age. Given the nature of Selective Service — deployed during times of threat — it weakens the “rite of passage” aspect; but during a genuine time of threat, volunteer forces are much higher in number.
If someone is in college, they are less likely to be chosen or even deferred entirely. In a non-universal university education state where education is often paid with loans or by working oneself to the bone, this leads to an easy way out for the upper-tiers of society because their foals may just have the education bought for them. This creates a larger pool of those from the lower classes who couldn’t even begin to afford college or pay off the loans as the default selected in the Selective Service. Those with the least to lose wind up fighting with their lives for those who don’t want to lose that they have.
Proposals have been made on modifying the Selective Service, but it has been kept in place in its current form for good reasons from the perspectives of those who make the laws. Altering it to include college students would take away the future educated they say, while the military already gives a college-level education to individuals based on merit and need rather than payment. Those who would be taken out of college would have their education considered when assigned their MOS. Altering it to include mares would be unfair because they form the future of society, aren’t on the front lines, and would cause difficulties in the co-ed formations. The current status is that mares are given leave if they get pregnant or if they are already on the front lines in a non-conventional war (and in a conventional war where conscription is direly needed, every pony is needed to help). Difficulties in co-ed formations can be diverted by all-female formations, or just biting the bullet and allowing co-ed frontline formations despite the fact of needless sacrifice to protect female comrades. At the very least, registered females with the Selective Service could be assigned the now defunct 1-Y classification (“Registrant available for military service, but qualified only in case of war or national emergency”) once assigned to those with medical difficulties.
Given the current status of military spending, and the size and capabilities of the Military of the United States, it is unforeseeable in the near future the need for a reinstatement of American conscription, but if it does happen, if things don’t change, ponies who fight and die for this country will be the same ones that this country would disregard in a time of peace.