Conscription is also called Selective Services or The Draft, Liberals and others disagree about it.
The case against
Drafted Soldiers are less effective than professionals
Drafted soldiers are generally less motivated than professionals, have less experience, and are less effective in combat.
Drafted Soldiers are more expensive than volunteers
Drafted soldiers are less likely to continue in service after their draft expires, forcing the retraining of a new group. It should be noted, however, that continuing privatization of the military is creating competition for highly trained soldiers between the military and private contractors which could eliminate this additional cost.
The Draft isn't equal
Wealthy and powerful elites have always been able to find a way to avoid the draft, and a new generation of Chickenhawks is always going to result. In the American Civil War, wealthy people could purchase a deferral from conscription for $300. In later wars, wealthy elites could influence law and enforcement of the draft to get their Children exempted or assigned to non-combat posts. It's always been the case that rich old men start wars for their own benefit and leave poor young men to do the fighting and dying. Now that neoliberalism has taken over, the elites have more wealth power and influence than ever, and efforts to change this will take a long time.
The case for
Many more traditional liberals believe that Involuntary Military Conscription (in the United States, The Selective Service System, SSS and the draft) is indeed constitutional. The United States Constitution gives the federal government the power to raise and maintain an army. More traditional liberals are likely to see involuntary or mandatory military service as a necessary part of free peoples commitment to self-government. Many believe that a fully voluntary military allows a class of society which is able to avoid service, have tremendous influence in the use of the military, stand to benefit from the use of the military and impact some parts of society particularly hard. In essence, a voluntary military creates a situation where the hardships of war are not felt by all people equally. Social elites are able to wage and benefit from war without the risk of their class actually participating in it. This situation is counter to the liberal belief of equal representation and equality before and under law.
The last two Republican Party candidates for President of the United States were able to avoid active deployment. President Bush was able to join an elite and highly selective unit of the Texas National Guard which he knew was not going to be deployed even though he was less qualified than other applicants. Mitt Romney was able to avoid the draft and potential deployment by serving on a religious mission oversees.
Historically, there has always been a group in the United States which preferred privilege and social hierarchy to equal liberty, and an opposing group which preferred equality under the law and a government of all the people, by all the people, for all the people. We are liberals, we prefer equal liberty, and accept responsibility as it's price. We don't expect that organizations will exist without hierarchy, but we do believe that people are equal in law and have equal rights, responsibilities, obligations and opportunities. The history of liberals in the United States is the story of fair-minded people working towards those goals, and the history of conservatives is of the powerful working to establish or reestablish rigid social hierarchies and the privileges of those at the top.
The use of force, and the threat of the use of force are essential tools to the establishment and maintenance of social hierarchy or of individual liberty. We Liberals understand that there are advantages to maintaining professional military. We also understand that a professional military has historically been the tool of the elites of social hierarchy. The maintenance of a large professional army is always difficult. The maintenance of a large professional army in peacetime is not only difficult, but potentially dangerous to individual liberty. Professional armies are not only expensive, but necessarily extremely hierarchical in nature. We all understand the expense, but few consider the danger of these hierarchies on civil society. The acceptance of rigid hierarchy by professional soldiers has a tendency to be disseminated into general society and erode our resistance to social hierarchy, corruption, and our commitment to individual liberty.
Involuntary Military Service, whether called conscription or SSS and the draft, has the goal of creating a military which is tied to all parts of society, so that risks as well as rewards are shared by all, in the hope that we all then realize the gravity of that dreadful business which is war, and not be manipulated into conflict which benefits the few at the expense of the many. .