Some compare this to the "islamic" invasions (of the Caliphate into Spain, or the Turkish expansion into Eastern Europe, who still remember that tyranny and are refusing to kow tow to Merkel).
But a better historica parallel would be the Goths fleeing from Attila the Hun.
From the Baldwin project:
The Ostrogoths (Eastern Goths) were conquered by this savage horde (i.e. of Attila the Hun). The Visigoths (Western Goths), stricken with mortal fear, hurried to the Danube and implored the Romans to save them from annihilation. For many miles along the banks of the river extended the panic-stricken multitude, with outstretched arms and pathetic lamentations, praying for permission to cross. If settled on the waste lands of Thrace they would pledge themselves to be faithful subjects of Rome, to obey its laws and guard its limits.
Sympathy and pity counseled the emperor to grant the request. Political considerations bade him refuse. To admit such a host of warlike barbarians to the empire was full of danger. Finally they were permitted to cross, under two stringent conditions: they must deliver up their arms, and they must yield their children, who were to be taken to Asia, educated, and held as hostages. Such was the first fatal step in the overthrow of Rome.Wikipedia article here.
all of this must be put into context: The Romans had settled barbarians before, but were overstreched and plagues and civil wars had weakened the empire.
There is one big difference, however: Few of the migrants (not refugees, because most are migrants) going into Europe are actual refugees, and few are women or underaged children.
Most are indeed young men who could be trained to fight, and some are probably trained terrorists.
But most are just men wanting easy jobs that pay well. If Merkel or Obama's open border policy was near the Philippines, we would undoubtably be sending them a couple million Pinoys to get jobs too.
The Goths, however, came from a background where they knew how to fight. Luckily most of these migrants do not know how to fight, although they could easily learn to do so.
The jihadi mind set gives them a slight advantage when they shoot or mow down innocent civilians who lack a gun large enough to stop truck, but in a one to one against people with some training, it means you lose.
Alas, like the weakened Roman empire, one doubts the young Germans know how to fight either. And that is the problem.
No one thought to put up concrete barriers to protect the market, even though France had suffered a similar attack on Bastille day, and despite the meme that "professional intelligence" cannot stop attacks, in this case and in several of the other cases, it seems that this guy had known terrorist ties.
As for Merkel's policy: if you don't know what happens when a million young men without women get the upper hand over people who they hate, one might suggest you read about the end of World War II....
update: SP on Iran vs Saudi