Cause is diminished when governments invade privacy and abolish freedoms without transparent processes.
Most people accept the "war on terror" is a regrettable necessity. There is a clearly discernible (if hard to quantify) terrorist threat; we are certainly entitled, indeed required, to protect the innocent against unjustified violence and our society against disruption and chaos.
We also recognise that what is at stake is more than lives and property. We are defending our way of life - the principles of democracy, the rule of law, the values of Western liberal civilisation.
So, we understand, too, that in fighting the war against terror, there is a risk that the weapons we use might jeopardise and betray the very values that we are defending. Sadly, there is already a long and shameful catalogue of just such instances.
The use of torture, including "waterboarding", is well established. The illegal imprisonment of suspects without trial or any legal recourse is still a reality at Guantanamo Bay.
The practice of "rendition" - the illegal kidnapping of individuals - was, and perhaps still is, commonplace.
It can be argued that these instances are not only at odds with the values we seek to protect; they are not even helpful in the war against terror. When the state itself commits what would in most circumstances - and certainly if committed by anyone else - be regarded as crimes, we cannot be surprised if the uncommitted are less than fully convinced by our protestations that we are taking a morally superior stance.
The illegal invasion of Iraq, for instance, was undoubtedly a major recruiting weapon for al-Qaeda. The West's involvement in Afghanistan, too, has meant that young Muslims have been more easily persuaded that the West is their enemy.
Our own societies, too, are weakened when we allow Islam in general to be tarred with the terrorist brush. The vast majority of law-abiding Muslims will naturally feel isolated and unfairly condemned by a blanket hostility to their religion.
These breaches of international law and affronts to basic common sense and morality should warn us that we need to be vigilant; and there is a range of less obvious denials of human rights that also demand our attention.
In the US, the UK and now in New Zealand, there is a growing tendency for the executive to excuse any invasion of individual human rights as necessary in order to identify and deal with the terrorist threat. Our security services increasingly claim the right to act in our name without any corresponding duty to explain themselves. And the courts in many Western countries show an increasing willingness to accept a certificate that national security is involved as a carte blanche for any activity that the executive undertakes.
The revelations by Edward Snowden show how far-reaching those claims can be. We now know that the right to privacy that most people imagine they enjoy is easily cast aside when the state decides that it needs to know.
We should bear these considerations in mind when assessing the Government's recently announced decision to withdraw the passports of a few New Zealanders who wish to go to Syria in order to join the fight against the Assad regime.
There is of course a long and honourable history of young men who have been willing to leave their homes and to fight in a foreign country for a cause they believe in. Thousands of young Britons and other Europeans, for example, joined the ultimately unsuccessful struggle to resist an emerging Fascist military dictatorship during the Spanish Civil War.
In more recent times, other overseas battles - often those whose purpose is to protect people from oppression - have been joined by volunteers. It is not as though the principle of intervention in a foreign struggle is regarded as wrong. The state itself has been happy to involve itself in foreign adventures, as countless invasions and interventions in other people's battles have shown.
The step announced by our Government in the Syrian case is a serious one. To deprive a citizen of the rights of citizenship, and in particular to deny the right to freedom of movement, is a major invasion of the freedoms that our society is thought to guarantee.
The removal of those freedoms has been carried out by the executive without any transparent process and on grounds which are unclear and not subject to scrutiny. Indeed, it is hard to fathom what the rationale for the decision is.
Has our Government decided to support the Assad regime against the opposition? Does it have the detailed knowledge to allow it to distinguish between the various elements of that opposition?
Does the apparent prohibition against involvement in the Syrian dispute apply to other foreign conflicts? Is the Government acting on advice from another authority and if so which? Does our Government claim the right to tell us which causes we are allowed to support and which not? If fear of "radicalisation" is the basis of the decision, what other "radical" causes would be similarly outlawed?
In the end, the claim that the threat of terrorism trumps all other considerations should be treated very cautiously. The war against terrorism must be fought, and fought hard; that makes vigilance to protect our fundamental values all the more necessary.
Bryan Gould is a former UK Labour MP and former vice-chancellor of Waikato University.