In one of the worst bursts of violence that this tense refugee summer has seen, Hungarian riot police responded on Wednesday to rocks, taunts and small fires set by agitated migrants at the border crossing here with water cannons, head-cracking batons and both tear gas and pepper spray.

Although the word was quickly spreading along the migrant trail that heading toward Croatia from Serbia was a better bet than trying to push through the heavily guarded border into Hungary, hundreds of straggling refugees continued to turn up at the crossing here in hopes that Hungary would change its mind and let them through.

But Hungary did not change its mind — prompting a grim demonstration of what can happen when an unstoppable force meets an immovable object.

And the demonstration is likely to continue as more migrants and refugees try to escape war and poverty in their homelands and find a new life in a continent that cannot agree on what to do with them. Already, the migrant trail was adapting, finding new ways to reach western Europe.

Tension had been building through the afternoon. About 2,500 migrants had set up camp along the narrow, two-lane road leading to the small crossing here — nothing more than a cluster of battered buildings and two lines of fence, topped with razor wire. …