A sudden mass crossing into Spain's North African enclave has strained Spain's borders and rattled the rest of Europe — bad news for Madrid and for open EU travel.

• About 50,000 people crossed from Morocco into Ceuta in roughly a day, and 34 drowned trying to reach it by sea.
• Ceuta sits in North Africa but belongs to Spain, so anyone who gets legal status there can move freely across the EU.
• Morocco appears to have opened its side of the border on purpose — a tactic it has used before in disputes with Spain.
• Likely triggers: anger over Spain warming to Algeria, Morocco's rival, and a Spanish court ruling that makes it harder to send back people who arrive by boat.
• France is tightening its border with Spain and Italy is floating a suspension of the EU's open-border rules.

Outlook: Expect more pressure on Spain's borders and growing talk in Europe of pausing free movement.