LONDON — Europe cannot simply pull up the drawbridge and begin intensive security checks similar to those administered by America's Department of Homeland Security in the wake of the Paris attacks, according to experts.
Greece has hundreds of islands off its coast, providing a multitude of options for those attempting to reach the West by sea.
Almost 750,000 migrants and refugees have made it to Europe this year — including nearly 220,000 people who arrived by sea in October, according to the United Nations refugee agency (UNHCR).
"We are fundamentally different [to the U.S.] in that regard," said Nick Whitney, the co-director of the European power program at the European Council on Foreign Relations. "Even if we wanted to go down that intensive Homeland Security route it simply isn't possible because of geography."
Under the Schengen Agreement of 1995, travel without passport checks is permitted within 26 countries on the European mainland. That means once migrants arrive in a country like Greece, they are free to make their way across huge swaths of the continent without facing border controls. At least one of the Paris attackers arrived in Greece on a migrant boat last month and later entered Serbia using a Syrian passport.
Friday's ISIS-linked series of attacks left around 130 people dead in the French capital and intensified calls for the reintroduction of border controls between member states.
Bruno Stanio, the deputy director of Human Rights Watch in Paris, said that while there may be "lone wolves among the sheep" most of the recent Islamist violence in Europe involved homegrown militants.
"They're not leaving Syria or parts of Iraq because they want to become terrorists," Stanio told NBC News. "They have been victims of terrorism, they've been victims of gross violations committed by murderous dictators. These are people who deserve protection."
Jeffery Rathke, the deputy director of the Europe program at the Center for Strategic and International Studies, said improved cooperation between intelligence agencies was essential to ensure extremists did not slip through the net or slide out of view.
"Free movement of people, which is of enormous benefit to European citizens, also provides the opportunity for people to seek shelter in a place where they may not come to the attention of the authorities," said Rathke, the former deputy director of the Private Office of the NATO Secretary General in Brussels.
He said that intelligence needed to be shared "not only of threats that may affect the nation where people are located but the possibility that threats could be carried out across the border."
Doug Ollivant, a senior fellow at the Washington, D.C.-based New America think tank, said that France was inevitably now facing a period of soul-searching.
"As is often the case with major terrorist events like this, the question now facing the French is how much security are they willing to accept without changing the character of the public space of Paris," he added. "They can do all kinds of security checks and increase surveillance, but they will have to strike a balance ... That's really the dilemma the French are going to be faced with."
The ECRF's Whitney said the Paris attacks brought back memories of the Irish Republican Army (IRA) — a paramilitary group that opposed British rule in Northern Ireland and carried out a series of bombings on U.K. soil in the 1970s and 1980s.
"We did everything we possibly could but at the end of the day it had to be a political settlement," he said. "There was no absolute way of hermetically sealing yourself off."
And if ISIS-linked terrorists want "to commit terrorist outrages they can activate people who have been in Paris or Brussels for years," Whitney warned.