In brief: During the Spanish Civil War (1936-39), the Republicans were fighting the Nationalists. The "Republicans" were the ones being overthrown, and were made up of mostly socialists, communists, republicans, anarchists, and left-wing militias (you can already understand how the factions turned against each other later). They received some arms from the Soviet Union. The "Nationalists" were the rebels, in that they were trying to overthrow the Republican government. Led by Francisco Franco, they were comprised mostly of Army officers, fascists, monarchists, and followers of the Church. They received many arms from both Mussolini's Italy and Hitler's Germany.
In 1936, at the outbreak of actual fighting, the Western Europeans stopped supplying the Republic and instead formed the "Non-Intervention Committee". The NIC was supposed to ensure that there was no outside intervention in the civil war. Needless to say, the NIC was a complete failure, as Italy and Germany continued to pour men and material into Nationalist Spain, eventually helping them to overthrow the government and win the war.
Syria, anybody? Kofi Annan's diplomatic efforts?
I see a few important similarities and differences.
First, there is no nation that is overtly pouring resources into the Assad regime's war effort like Italy/Germany was into Spain. Iran is a possible contender though it's not to the same degree and the reports concerning the extent of Iranian intervention are mixed. However, the similarity is disconcerting.
Second, while much of democratic Europe was staunchly against intervention in Spain, there is the feeling that a search for acceptable and productive intervention in Syria is ongoing. A note of caution: that desire for "progress", and the willingness to cheer any move -- however small -- towards an eventual end to the killings, can hide the actual lack of anything being accomplished, outside of a continuation of mass murder.
Lastly, the NIC was never successful in establishing a blockade around Spain to prevent the influx of guns and material. Actually, it never really tried. It was merely a diplomatic association. In fact, the Soviet Union, Germany, and Italy were members of the NIC! In Syria, it's basically the same thing. Has the Syrian-Iranian border been secured? What about the Turkish-Syrian border? Nope. Then again, there is no call for a blockade of Syria, though that seems to be implied. What I mean is that if there has been no agreed-upon way for multilateral intervention in Syria, one would expect that there is no intervention happening in Syria -- which is probably not true. Saying "we will not arm the rebels" doesn't mean that there isn't somebody else arming the rebels (or the regime). Also, "we will not arm the rebels" doesn't mean "we will prevent any effort to arm the rebels."
My hope is that in Syria, those who have died will not be forgotten, Assad will not be let off the hook, and the international community will facilitate/impose a political transition. The Non Intervention Committee never had similar goals in Spain, which gives me hope.
In closing, consider this quote, which was issued by the XXXII Peace Congress after the civil war ended (Wikipedia article):
"Congress considers that a policy of non-intervention, or of abstention, is shown to be insufficient in principle and in practice dangerous, for it paralyses those states which obey it and becomes advantageous to those which violate it."