REMEMBER the so-called "coalition of the willing?" The Bush administration was fond of using the term when referring to countries who supported the 2003 invasion of Iraq.

Today, as Washington again finds itself embroiled in Iraq fighting the Islamic State (IS) group, a new phrase has been coined to describe those allies America seeks to engage. They have now been dubbed the "coalition of convenience".

Washington has a tough job bringing together regional allies to combat IS, whose threat is now beyond doubt.

Already the jihadists have etched their brutal mark on the region by murdering thousands of unarmed Iraqis, threatening minority groups with genocide and beheading Westerners, including a Scot. They also have at least two other British hostages, one of whom they released a video of yesterday.

Some analysts, sceptical of Washington's regional allies, have been scathing about efforts to pull them into action against IS. Writing in the influential US-based Foreign Policy magazine, Aaron David Miller recently described it as a "coalition of the semi-willing, the constrained, and the self-interested".

But putting regional concerns aside for the moment, just how much of a threat does IS pose as a transnational terrorist organisation?

Yesterday, the range of its potential threat was brought home when Australian authorities raided homes in Sydney and Brisbane in an unprecedented counter-terrorism operation after intelligence reports indicated IS terrorists were planning random "demonstration" killings, by beheading members of the public.

Like many countries across the world, Australia is concerned over the number of its citizens believed to be fighting overseas with Islamist groups, including a suicide bomber who killed three people in Baghdad in July and two men shown in images on social media holding the severed heads of Syrian soldiers.

From Asia to Europe and North America, many governments now share Australia's concerns, worried that fighters will return home and commit acts of terror. Many are now scrambling to intercept these potential combatants. Certainly if numbers are anything to go by, there is real cause for concern.

Just a few weeks ago the CIA revised its official estimate of the total number of IS militants in Iraq and Syria upward almost threefold to between 20,000 and 31,500 combatants.

Most worrying is that an estimated 12,000 of these who have successfully joined the IS and other jihadist groups in the region are foreigners, according to the CIA.

Of these, 2,000 are Europeans, the EU estimates. The UK government alone says some 500 of its citizens - including the suspected killer of the American journalists James Foley and Steven Sotloff and Scots aid worker David Haines - have joined militant groups in the Middle East.

Some countries elsewhere in the world, though, have been identified as even more potentially fertile recruiting grounds.

Indonesia, for example, is a particular risk. The southeast Asian nation of roughly 250 million people has the world's largest Muslim population and more Muslim men under the age of 30 than any nation except Pakistan. Nearly 80 per cent of the country's internet users are plugged into social media, which has proved to be the key tool for IS recruitment.

Hardy surprising, then, that IS propaganda, urging Indonesians to support the extremists cause in the Middle East, has spread throughout the archipelago, despite concerted efforts by the government to contain the threat.

The terrifying terrorist capacity a fully fledged IS global network could unleash was chillingly highlighted recently by the discovery of a laptop computer from an overrun IS base in northern Syria.

Handed over by a moderate Syrian rebel group to intelligence analysts for examination, the computer's hard drive was shown to have 146 gigabytes of material, containing a total of 35,347 files in 2,367 folders, which included documents in French, English, and Arabic. Among these were the usual manuals on how to construct bombs, steal cars, and create identity disguises for militants moving clandestinely from one location to another.

Most revealing of all however, were files showing that the laptop's owner was teaching himself about the use of biological weaponry, in preparation for a potential attack that would have had global implications.

Nothing on the computer showed the terrorists already possessed such weapons, but the very fact the research and planning was at such a stage had alarm bells ringing at spy headquarters across the region and beyond.

Despite these fears, right now it would be wrong to compare today's IS global network and cell structure with that of al-Qaeda's at its height in 1999. For the moment, IS's main threat is on the open battlefield and, to that end, some kind of military coalition of the kind Washington is trying to construct can contribute substantially to thwarting the jihadists advance.

The battlefield prowess of IS is now well established. Should that extend to the development of an equally efficient clandestine terrorist network, then counter-terrorism agencies will face a considerable challenge.

Above all, their efforts will necessitate the sharing of vital intelligence. And if one thing has long since been established, it is that intelligence communities worldwide could never be described as a coalition of the willing.