Cameron and Obama are attending the NATO summit in Wales where they are discussing the rise of IS in Syria and Iraq and the rising conflict between Russia and Ukraine with other military leaders.
They called on NATO leaders to work on building relationships and to promote stability in the region so Iraq and Syria can resist IS. Specifically, they said it is essential for the Iraqi government to be more inclusive of the Sunni, Shia, Kurdish, Christian and other minority populations. They also condemned the murder of two American journalists by IS jihadists.
"If terrorists think we will weaken in the face of their threats they could not be more wrong. Countries like Britain and America will not be cowed by barbaric killers. We will be more forthright in the defense of our values, not least because a world of greater freedom is a fundamental part of how we keep our own people safe," wrote the two leaders.
Obama and Cameron touched on the topic of Russia, calling for a NATO military presence in Eastern Europe to defend allies against Russian "force" and to invest in military installments.
"We must use our military to ensure a persistent presence in Eastern Europe, reassuring NATO members in Eastern Europe and making clear to Russia that we will always uphold our Article 5 commitments to collective self-defence. And we must back this up with a multi-national rapid response force, composed of land, air, maritime and special forces, that could deploy anywhere in the world at very short notice," read the op-ed in The Times of London.
The NATO summit is being held in Wales Thursday and Friday.