One of the disasters of the 20th century Cold War, the Iron Curtain, did two things: it kept the people of eastern Europe from migrating to the West; it taught how to build a wall (or fence, or other frontier control) that is marked by its efficiency, second only to its cruelty. So when those traipsing through Serbia meet the construct being realised by the Hungarian government they are in for a shock.
Hungary has decades of experience on which to draw to make their physical defence of their country effective. So has Germany (at least the former GDR), not to mention the Balkans. Walls are social structures too, and the societies that were once shut in have not lost the capacity to run an effective barrier to shut people out - this time with brio rather than the resignation of just doing a job.
The UK gave up on its moat, its coastguard, its navy and its long-standing policy opposition to mass immigration under the 1997 and later governments. Other Western European states had large migratory flows as well - Italians to Belgium, to Germany, for instance and, while the Turks to Germany wasn't exactly European, guest workers were useful and temporary. Temporary was the key word. All these European kinds of migrations were reversible as soon as work disappeared, and when out of work benefits were so unenticing. The UK, however, experienced a very different kind of migration; a migration of permanent settlement (as did France, though for different reasons, and earlier). So awful were their countries of origin in terms of opportunity, democracy, cultural normality, that the entrants never return - not permanently. All Italians go home. You would, wouldn't you? Turks didn't even have the choice until fairly recently. But what happened to the UK and France is now threatening to be the norm for Europe.
Those who know how to build a wall (in its full meaning) are going to do so. And maintain it. Meanwhile the transit states, like Italy and Serbia, are joined with the migrants from elsewhere in pushing relatively undefended or inexperienced countries, or countries with large, extant extra European culture settlements to cease even attempting to reinstate border controls against not temporary and desired labour resources but against permanent and unskilled settlers wholly inappropriate to European needs or wishes. Eastern Europe knows what to do, and has perfectly reasonable and acceptable migration patterns for its east European nationals. It's no good trying to turn transit countries into lagers either; these transit countries too have reasonable, settled migration patterns and no provision for settlement of extra-European migrants.
Hence the scenes at Calais, Menton, the Alpine crossing points into Austria and Germany: no experience of keeping people out in continental Europe, and the poor choices made in 1997 in the UK, have now to be remedied.