Image copyright AP

After the parliamentary vote in favour of taking action against Islamic State militants, a phoney war kicked off.

RAF Tornados conducted at least five missions over Iraq, and returned five times with their bombs intact, before finally striking twice in northern Iraq on Tuesday.

Why, after more than a month of British reconnaissance flights, access to allied intelligence, and intense fighting on the ground, did Britain appear to be struggling to find targets?

First, Britain's legally and politically-driven decision to strike only in Iraq, and not Syria, means we cannot strike Islamic State's (IS) main political and economic infrastructure in northern and eastern Syria, nor participate in the much-belated effort to relieve Syrian Kurds in the town of Kobane.

Image copyright Getty Images Image caption IS fighters attacked the border town of Kobane

The contrast between intensive US-led bombing across Syria on the one hand and anticlimactic British sorties in Iraq on the other will have been apparent to all.

Second, the nature of targets differs between the two countries. In Syria, the US and its Arab allies struck mostly at fixed sites, such as IS-held military bases, command hubs, oil facilities and gas plants.

In Iraq, the US - and more recently, France - has been striking tactical battlefield targets, including massed units of IS fighters and their vehicles, often in close co-ordination with Kurdish ground forces.

On Friday and Saturday, for instance, the coalition destroyed an airfield, a garrison, and a training camp in Syria, but only four armoured vehicles and a "fighting position" in Iraq.

More than 92% of all airstrikes since August have targeted vehicles, overwhelmingly in Iraq. These Iraqi targets are often smaller, easier to conceal and more mobile than their Syrian counterparts - and therefore much harder to identify, track and destroy.

In choosing to stick to Iraq, Britain has signed up for an operationally more difficult mission. Moreover, IS has had a long time to prepare for airstrikes and will have spent the past month dispersing and concealing its forces, having learnt lessons from the first wave of US airstrikes that began in August.

Indeed, it is easy to get it wrong even with immobile targets. On Monday morning, the coalition reportedly struck a base that was abandoned by IS several months ago.

And so while British aircraft might previously have identified an IS unit or building in a particular location, the coalition will have to find them again.

Third, these targeting challenges mean that airstrikes often require so-called forward air controllers - personnel on the ground who can precisely designate a target for aircraft. Three years ago in Libya, for instance, SAS forces helped guide Nato airstrikes onto Libyan military targets.

Image copyright Reuters Image caption US Air Force Strike Eagles flying over northern Iraq after conducting air strikes in Syria

Although British and allied special forces have been present in Iraq for more than a month, it is possible they are only now taking a more aggressive position, deeper inside Iraq, after parliamentary authorisation.

Fourth, the government's sensitivity to civilian casualties is likely to be exceptionally high, both because of domestic political concerns and concern over pushing Iraqi Sunnis further into the arms of IS.

The coalition will be aware that misplaced strikes will risk the Arab military participation in the coalition, help recruitment to IS, and undermine the broader political effort - peeling Sunnis away from IS.

This largely rules out early strikes inside IS-held cities, at least until the intelligence picture develops further and local ground forces are in a better position to take advantage.

It also means Britain will be loath to drop bombs on congested battlefields, where they run the risk of killing allied forces - Kurds or Iraqi soldiers - rather than jihadists.

Image copyright AP Image caption Iraqi Shia militiamen are trying to stop IS from reaching Baghdad

Fifth, Britain's coverage will be relatively limited, given that we have only devoted six Tornado jets to the mission - the same number as Belgium, and fewer than Denmark or Australia.

This small contribution is itself a reflection of our shrinking air force, which only has seven combat-capable squadrons compared to more than four times this number 25 years ago.

Eight Tornados are already deployed in Afghanistan. Even adding in Britain's surveillance aircraft, this pales in comparison with the array of firepower the US has deployed, including drones, cruise missiles and five different types of combat-capable aircraft.

But these targeting challenges speak to a deeper issue: airpower alone, without effective and co-ordinated ground forces, is a very limited instrument, even with months of careful intelligence collection and total control of the skies.

Nearly two months after the US first began airstrikes in Iraq, IS continues to advance.

To give only one example. Within the past two weeks the group has been able to conduct a five-day siege against an Iraqi Army camp north of Fallujah, eventually massacring between 100 and 500 soldiers.

Such incidents will recur and it will be hard for Western nations to explain why, after all the political noise, there are no quick military solutions.

But the coalition has accepted this will be a long war, measured in months or years rather than weeks. They therefore see no rush to blow things up - slow and steady, rather than shock and awe.

Shashank Joshi is a senior research fellow of the Royal United Services Institute (RUSI) in London.