Justin Welby has announced his support for UK air strikes in Syria, telling the House of Lords today that the criteria of a just war have been met.
Speaking in the House as MPs debate the issue today, the Archbishop of Canterbury said: "The just war criteria have to my mind been met, but while they are necessary, they are not by themselves sufficient in action of this kind".
He warned that the UK could "end up doing the right thing in such a wrong way that it becomes wrong thing."
Aerial bombing "plays into the expectation of ISIL and other jihadist groups in the region, springing from their apocalyptic theology," he added.
"The totality of our actions must subvert that false narrative – because by itself will not work. If we act only against ISIL globally, and only in the ways proposed so far, we will strengthen their resolve, increase their recruitment, and encourage their sympathisers.
"Without a far more comprehensive approach, we confirm their dreadful belief that what they are doing is the will of God."
Welby called for a holistic approach to the war in Syria, insisting that "there must be a global theological and ideological component...to what we are doing".
"It must be relentlessly pursued and promoted, and it must include challenging Saudi Arabia and Qatar – whose own promotion of a particular brand of Islamic theology has provided a source from which ISIS have drawn a false legitimisation," he said.
"It must also show clear support for global mainstream Muslim and other religious leaders...Only a holistic, theological and global policy will achieve our aims."
The Archbishop also called for "greater generosity" in the UK's response to the refugee crisis. That hospitality "must be accompanied by a clear strategy that reduces the need for others to seek sanctuary... and enables those who have fled to return," he said.
"The communities that have lived there for 2,000 years should not simply be emptied from that region."
Welby's comments are likely to be supported by other Church leaders. A statement released by the Archbishop of Westminster Vincent Nichols, head of the Catholic Church in the UK, today said British air strikes could be "defensible".
"Effective action is necessary to stop the grave harm being inflicted by ISIS on civilians. While indiscriminate violence is never justifiable, specific use of force to protect the vulnerable is defensible, if it is combined with sustained diplomatic and humanitarian efforts," Nichols said.
"As Pope Francis has said: where there is unjust aggression, it is licit to stop the aggressor."
Nichols referred to a trip he took to Iraq in April of this year, during which he concluded that "proportionate military intervention" is necessary to stop ISIS and other terror groups.
A joint statement released today by The Baptist Union of Great Britain, the Church of Scotland, the Methodist Church in Britain and the United Reformed Church was less supportive of military action in Syria, however.
It said that aerial bombing "is unlikely to have a decisive role in defeating Daesh", would likely cause significant civilian casualties, and that the churches are "convinced that Daesh can only be defeated through a comprehensive economic, diplomatic and security strategy".
Though the statement praised the UK's efforts to suppress acts of terrorism, it said that the assertion that British military intervention in Syria "will reduce the terrorist threat to the UK...requires significant qualification".
"Ultimately a more vigorous campaign across the Middle-East as well as in our own country is vital to counter the ideology of Daesh and the propaganda that endorses religious and sectarian violence," the statement added.
"Western military intervention in the Middle East is unhelpful in that it makes sharing on matters of peace, equality and the common good more challenging."
It also said: "Our churches eschew all forms of intolerance and hatred and urge that measures taken to overcome the intolerance demonstrated by Daesh must also seek to promote values that we hold dear."
The UK government published a motion on Tuesday saying that ISIS poses a "direct threat" to Britain, and there is a legal basis for action. MPs are currently debating the issue in the House of Commons, and will vote at around 10pm this evening.
Prime Minister David Cameron today said the air strikes would help to "keep the British people safe" from terrorists.
"Do we work with our allies to degrade and destroy this threat and do we go after these terrorists in their heartlands from where they are plotting to kill British people, or do we sit back and wait for them to attack us?" he asked MPs.
"The question before the House today is how we keep the British people safe from the threat posed by ISIL. This is not about whether we want to fight terrorism, it's about how best we do that."