as to the Vietnam War, as long as over 50% of the people supported it, the draft worked out. Once 50% of the population OPPOSED the war (Only starting in the Summer of 1968) did the draft "bad side" came into play. That "Bad side" was increased opposition to send draftees to fight an unpopular war AND the men drafted brought with them an deterioration of the troops (The US Army by 1972 had become for all practical purposes non functioning, which was chief reason for the pull out from Vietnam of all ground forces).
Vietnam is NOT the only time this had happened, During the Third Punic War the Roman Army took three years what their grandfather's (Who fought Hannibal) could have done is three months. Why? The Roman Soldiers OPPOSED the war for all its meant was increase wealth for the Roman Elite while they did all of the fighting and dying. Maius "Corrected" this problem by switching to PAYING the troops, the start of the Roman Mercenary Army (which ended up forming the first Political dictatorship of the Roman republic, that of Sulla, who is the process of gaining power would kill Maius) those same "Volunteers" (paid mercenaries) would later be the basis of Julius Caesar raise to power and the foundation of the Roman Empire.
The problem with Draftee armies is NOT restricted to Ancient Rome or the US war in Vietnam. Britain from the late 1600s till WWI always used "volunteers" for the simple reason then the British did NOT have to worry about the support the war had back home (Even if the people opposed it, since the people could rationalize the deaths of the soldiers as they "Choice" since they all "Volunteered" to serve. France, Germany, Spain and Italy all had "Volunteer" units during the rush for Africa in the late 1800s. None of them dare send in their draftees to grab these third world Colonies, the people back home would OPPOSE the war and refuse to permit their sons to go, but would permit "Volunteers" to go even if the war was opposed by most of the people of the Country (Which was the case in most of Europe during the "Rush for Africa", contrary to popular opinion).
The End of WWI, shows the control the Draft can have on a war. Russia went into Revolution, where much of the Draftees of the Czar's Army no longer supported the war. The same thing occurred in Hungary and Austria. In Germany both the Navy and Army enlistees mutinied to end the war. Britain and France were NOT in that much better shape. The Army of France went into full scale mutiny in 1917 (The Allies ended up using American troops to be the spearhead of any French attacks in 1918, given the low level of faith the French High Command had in their own troops by the end of 1917) after reinforcing those same French Units in 1917. England was not in that much better shape in 1918, they army had NOT revolted, but Britain only had the draft since 1915, not since before the war as in the case of France, Germany, Austria-Hungary and Russia, but even the British had problems with their troops. So much problem that when Germany agreed to surrender under Wilson's 14 points, France and Britain gladly agreed do to how bad their armies were becoming in late 1918. The subsequent Peace Treaty was a victor's peace as to Germany, Austria, Hungary, and Turkey but by the time the full terms had been agreed to the Armies of Germany, Austria, and Hungary had been dissolved and all three countries were in the mist of Communist revolutions (Which all three countries did put down but with new "Volunteer" forces NOT the former Draftee units).
The reason the revolutions at the end of WWI occurred was the Draftees wanted revolutionary change as while as the people, since the draftees reflected what the people wanted (for the people and the draftee army were the same people). We saw this again as the Collapse of the Soviet Union and the Warsaw Pact nations. The Draftee Armies would NOT support the Communists in power for the people of Russia and the rest of the Warsaw Pact no longer supported the Communists. In the only Warsaw Pact Nation to have an Armed fight in their revolution, Romania, it became a fight between the Army made up of Draftees and the All Volunteers of the Secret Police (The Romanian Army won, for they had the support of the People). In Russia the attempt to overthrow Gorbachev failed when the Draftees of the Army refused to help the coup plotters (The Soviet, later Russian, Army subsequently had to support Yelsin for the simple reason that is who the Draftee trusted).
Back to Vietnam and the Soviet intervention into Afghanistan. Both were popular in the US (as to Vietnam) and the Soviet Union (Soviet Intervention in Afghanistan) until some years after the intervention (Four years for Vietnam and the US, about five years for the Soviet Intervention in Afghanistan). Once that opposition occurred, both countries had to pull out or see a rapid deterioration of their Army. The US Army was almost destroyed by 1972, it still had the best supply line, the best equipment, but the troops were in poor condition to fight. As to the Russians in Afghanistan, a similar decline took place, and once the Soviet Army reached about the same level, the Soviet Union pulled out (and this was BEFORE the collapse of the Soviet Union, Communist Afghanistan would outlast the Soviet Union by almost two years, but Soviet Troops were out at least two years before the Soviet Union fell.
I mention the above for a Draftee army provides the best type of army AS LONG AS THE PEOPLE WHO THE DRAFTEE IS COMING FROM SUPPORT THE WAR. On the other hand once the people oppose a war, so will the draftees who will either avoid being drafted OR if draftee just go through the motions required of them.
On the other hand an All Volunteer Army will fight long after the people has rejected the war. The present Wars in Iraq and Afghanistan is the classic example. The Majority of Americans opposed both wars from the Beginning (With some majority support to attack Afghanistan if that was to get bin Laden, but once bin Laden was NOT found and turned out NOT to be the main military objective of the war in Afghanistan, the majority of Americans even opposed that war). It has been almost ten years of fighting (Ten years will be on October 7, 2011, US attacks on Afghanistan started on October 7, 2001). The US intervention in Vietnam less then nine years years (US Troops withdraw after the peace treaty of January 1973). The Soviet intervention in Afghanistan was just under ten years, December 1979 till February 1989.
Soviet Afghanistan Intervention:
http://en.wikipedia.org/wiki/Soviet_war_in_AfghanistanVietnam war:
http://en.wikipedia.org/wiki/Vietnam_WarUS Intervention in Afghanistan:
http://en.wikipedia.org/wiki/War_in_Afghanistan_(2001%E2%80%93present)
Just look at Vietnam and Afghanistan. In Vietnam with a draftee army, the US Was out within five years from the time the Majority of Americans opposed the war. The Soviet Union pulled out about almost as quickly once the war no longer had popular support in Russia. The Majority of Americans HAVE Never supported wither war, and here we are still fighting then almost ten years later. That is the difference between a draftee and volunteer army, a draftee army is only good if the people being drafted support the war, once that is no longer the case, the war must end. On the other hand a volunteer army will fight even as the people of the country they are from oppose the war. Thus you have a greater chance with fighting a war with an all Volunteer Army then with a draftee army.
Side note: Many right wing sites have adopted the idea that All Volunteer Armies are better then Draftee armies. That has NEVER been the case. Most of the example they tend to cite, tend to a well equipped, well supplied Volunteer army against a poorly equipped, poorly trained, poorly supplied draftee army. In such cases the better supplied and equipped army will win. On the other hand, if both armies are well trained, well supplied and well equipped, the draftee army tends to win. The reason for this is simple, when armies can selected who can go to the army, as oppose to taking those people who are willing to volunteer, they tend to take in better educated, and higher intelligent people.
During WWI, one of the comments was many of the NCOs of the German and French Armies could be Officers in the British Army, for they had the intelligence and education to be an officer. This is how much higher quality you had in the general enlistee ranks do to the Draft.
During WWII, the French Free French Forces, relying on the better quality a draft gave them in NCOs, continued the French practice where many jobs and functions done by US and British Officers were instead done by French NCOs. The reason for this was before WWII the US and British Armies were all volunteer forces, but the pre-war French Army was all Draftee except for French Marines and the Foreign Legion. This ability to use NCOs in place of Officers saved enough resources, that when the US agreed to fully equipped some Free French Forces, the Free French were able to form several additional battalions of Infantry do to this savings. I.e. the Free French Forces, since they had NCOs doing what in the US Army was done by Officers, when given the resources and equipment of a US Division had enough excess resources (since the French Division had a lot less officers, given how much of the work was done by NCOs) that the Free French were able to save these resources and use them to to raise additional battalions.
Just pointing out the main problem with a Draftee Army (and I am using that term in reference to a Universal Military Service Army, not draft to fill in an other wise "volunteer" force) is it tends to reflect the will of the people. Even the Kaiser recognize this during WWI and made effort to get the opposition Social Democrats to support his war effort (and when the Social Democrats backed the mutiny in the Fleet and the Army, WWI ended, and the Kaiser had be abdicate). The French Government did the same, as did the Russian Czar (Which lead to the overthrow of the Czar when it became clear Russia was losing the War and it needed to end). Britain made such effort in England, Wales and Scotland and even tried in Ireland (and it was working, even in Ireland until the British Army blow the program when it sentenced the Irish who lead the Easter Rebellion to death and then carried out the sentence, this turned Irish popular opinion against the British Army and that lead to the subsequent Irish Civil War). If WWI had NOT ended when it did, both French and Britain may have sued for peace themselves (In fact in 1938 Winston Churchill, WWII British Prime Minster, and WWI Lord of the Admiralty, told an American Reporter that had the US NOT entered WWI in the Spring of 1917, the parties would have had a peace treaty by the end of 1917. The American reported who reported this statement later, under oath when asked by Congress, said yes, Churchill made that comment, through Churchill after the story came out in 1838 said he did not, but refused to say so under oath). The Draftee Armies of both Britain and France were at their limit in 1917, as were the people in both countries. us why most historian believe Churchill did make that statement. The Volunteer Army had failed Britain in 1915 (Do to the lack of Volunteers more then any other factor) and the Draftee armies of French and Britain were on the edge in 1917 (The French Army more then the British, but the French had suffered most of the battle losses on the Western Front from 1914-1917 and this would continue in 1918, but with US troops providing the spearhead of most French attacks).
The key to quality troops tend to be the commitment of the Soldier to the Cause. Draftees can have the same commitment as a volunteer IF THEY BELIEVE IN THE WAR, if the draftee does not, he will be a poor soldier. Since draftees tend to be higher quality soldier material, draftee armies tend to be superior to Volunteer armies, if the draftee in committed. If there is no commit to the cause the army is fighting for, then volunteers are better. Thus why Britain used a small volunteer Army, French had its French Marines and Foreign Legion to do what the British Army did in what we now call the Third War. Germany and Italy also followed that French policy for their colonies, for the same reason, opposition to the war has less affect on a volunteer army unit then a draftee army unit.
One last comment. While the Post-WWII French Army was Draftee, those draftees were rarely, if ever sent overseas, even while the French were involved in Vietnam from 1946-1954. By 1954 the All Volunteer French Forces, mostly Foreign legions and French Marines, had been defeated by the draftee army of what became North Vietnam. By 1954, even these French Volunteer forces were on they last leg, they were in worse shape then the draftee US army would be in in 1973. Another all volunteer army the Right wing does NOT like to cite when pointing out volunteers are better then draftees.