“Surely "greatest mistakes".
Now, for example, if Germany decide to take in 800,000 immigrants, they then ALL have the right to then come over the UK to live if the want to, and we cant stop them”
This has nothing to do with the Schengen Agreement, VHG.
Before Schengen free movement of goods and people was an enshrined principle in the EU. People had the freedom to move between EU nations but border checks were still in force (though in practice they had been all but abandoned in may places especially in western mainland Europe). This meant that people could be challenged when attempting to cross borders but if they were citizens of an EU state they could not be denied entry. Schengen changed that. The already porous borders that did exist on mainland Europe were completely and formally abandoned.
So the introduction of Schengen made no changes to the ability of people with EU citizenship to be able to travel to any EU country. Even without Schengen the UK could not deny entry to any EU citizen. What Schengen did do was to abandon internal borders meaning that people not entitled to be in the EU at all were free to roam across the Schengen area. Without Schengen they could have been stopped at national borders (though were unlikely to have been so).
What this means is that there is no necessity for countries receiving migrants to spend any time attempting to process them. They can simply usher them across their borders to other EU nations.
The two greatest achievements that the Euromaniacs spout is the single currency and the Schengen Agreement. Nothing needs to be said about the first except that the dire warnings of its potential problems, made by people with more expertise than the politicians who implemented the folly were brushed aside. But similarly the Maniacs were adequately warned that the Schengen Agreement, apart from providing free movement for people entitled to be in Europe, would do the same for those with no entitlement to be here. Those warnings were similarly brushed aside.
Discounting some decisions taken in association with the two World Wars, the Schengen Agreement was the second biggest political folly (after the single currency) taken in Europe in probably a hundred years. The results of both these projects are plain for all to see. The probability of these failures were well forecast. But still the EuroNutters insist they were right.