The breakdown of the Abbott government’s cabinet process continues apace. Tony Abbott leaks proposals to media mouthpieces before raising them with his colleagues. Barnaby Joyce is openly at war with the government over the approval of the Shenhua mine in NSW. And now, two senior members of cabinet are at odds over the extent of the threat posed by Islamic State, following Malcolm Turnbull’s speech on Tuesday night.

Julie Bishop, who has previously said Islamic State was the greatest threat in recent world history, is plainly embarrassed by her friend and colleague’s suggestion that the government’s overheated rhetoric has lent credibility to the terrorist group. She responded to Turnbull yesterday by claiming her views on Islamic State reflected what she had been told in “classified briefings”.

This is a regular theme of national security discussions in Australia: politicians make assertions they expect to be accepted at face value and then, when challenged, invoke unspecified intelligence information to which only they themselves are privy.

This was the basis for Australia’s involvement in the monumental debacle that gave rise to Islamic State: the invasion of Iraq. Whatever right policymakers and intelligence agencies had to be taken on trust was forfeited when they lied to Australian citizens about Saddam Hussein having weapons of mass destruction.

If Julie Bishop has evidence for her claims about Islamic State — and evidence that Turnbull is wrong — she should produce it. Without that, voters have very good historical reasons to take her claims with a grain of salt.