WESTERN democracies, including our own, are grappling with the question of how to respond to potential terrorism while protecting civil liberties. The word "potential" is key here in that the response to actual terrorist action is the same as to any other serious crime: police interdiction followed by judicial process.
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When health professionals who had volunteered to go to Africa to fight Ebola returned to their home country, they were subjected to a quarantine for a period of time. It turned out that an obligatory quarantine for the asymptomatic health worker was unnecessary but it was perfectly reasonable to argue that public health requirements for those exposed to a serious infective illness overrode their rights to liberty at least for a limited time.
People who travel under this or any other democratic country's passport are exercising a privilege, not a right. Generally, joining armed conflicts other than ones sanctioned by the home government may be grounds for revocation of citizenship, certainly grounds for conditional revocation of passport privileges.
For any traveller, to accept and travel under a passport is to accept the conditions of the issuing government for its use. Those who choose to travel and to become involved in association with known and designated terrorist groups are liable for a number of possible measures that may limit their full freedom, on grounds similar to public health measures to contain any form of widespread hazard to public safety.
If it can be established that a returning traveller has committed a crime by actively participating in unlawful armed conflict, there currently exist sufficient criminal justice avenues for prosecution and punishment if necessary. What is at issue is the consequence of mere travel to an unauthorised armed conflict. A public health approach, taking account of the hazard such a person may pose, would entail a continuing monitoring, much as in parole conditions, for a period determined judicially. Anyone who travels without official authorisation to a conflict zone needs to accept there will be consequences.
That a small number of people may pose such a public health threat to our common safety does not provide justification for widespread surveillance on ordinary citizens, most of whom would happily tend to their own business and would wish to live free of fear and its consequent inhibitions. Government ought not be allowed to limit the freedoms of the law-abiding many in order to counter the fear posed by a few.