Why Does France Control The UK's Border?

Chaos at the UK-France border is exposing problems that could impact Britain's potential EU exit. So why is the Calais border in jeopardy?

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Why is there a crisis in Calais? (bbc.com)
"Attempts by migrants to cross the Channel from France into England continue unabated, causing delays and disruption to train services."

French minister: Brexit would threaten Calais border arrangement (theguardian.com)
"A senior French government minister has said his country could tear up an agreement allowing British border police to operate in Calais if the UK leaves the EU."

Migrants in Calais Desperately Rush the Channel Tunnel to England, Night After Night (nytimes.com)
"The sun had barely set when a 23-year-old Eritrean woman who gave her name as Akbrat fell into step with dozens of other men and women and started scaling the fence surrounding the entrance to the French side of the Channel Tunnel."

Agreement between The Government of the United Kingdom of Great Britain and Northern Ireland and the Government of the French Republic (gov.uk)
"In the present Agreement the term "officer" shall mean exclusively officers of the French Customs and officers of the French National Police belonging to the Direction Centrale de la Police aux Frontières."