Iain Duncan Smith is a former Secretary of State for Work and Pensions, founded the Centre for Social Justice, and is MP for Chingford and Woodford Green.

As he takes up his job at the Ministry of Defence, the new Defence Secretary faces some enormous challenges. What follows are some thoughts on the scale of those challenges.

His first challenge will be sorting out the MoD budget. The NATO commitment to a minimum level of spending of two per cent per year has proved difficult for the Government, and although the figures show we are just above the target, it has put a strain on resources.

To put it in context, the 2015 Conservative manifesto committed to increasing the defence equipment budget by one per cent above inflation, and the 2017 manifesto changed this to wider 0.5 per cent above inflation increase for the defence budget.

There have been a number of estimates of the financial issues at the MoD, and the deficit could be as high as five per cent of projected spending commitments over the next ten years. This needs to be sorted out with the Treasury at once, or the MoD will have to make further cuts in current spending which will stretch the Armed Forces to breaking point.

Whilst much has changed internationally since the Strategic Defence and Security Review was published in November of last year, nonetheless the principal global threats to the UK’s interests have not changed significantly. That is why the Defence Secretary should recognise that, based on the nature of world affairs, the 2015 SDSR should not need a wholesale reassessment or replacement.

Such a decision has an impact on acquisition policy. The problem is that defence acquisition is unlike other areas where the government buys equipment and supplies. This is because it is not only an investment in the security of the UK but also often in the well-being of those who defend Britain, who rely the MoD getting it right as their very lives depend on it.

However, this priority bumps into another strategic issue: the need to keep a strong defence industry in the UK, which cannot be sustained on British orders alone. This means that defence exports are critical, and we have just seen what happens when those exports dry up as BAE have had to lay off some of their workforce.

Therefore procurement decisions become political, and the Defence Secretary will have to navigate the thorny matter of defence reliant jobs in MPs’ constituencies.

Underscoring this is the MoD’s failure to manage their procurement programmes and losing large sums of money in the process. One historical example of this was the sudden cancellation of the Nimrod MRA4 which, as a result, left the UK without any maritime patrol capability.

This decision arose not just because the contractor failed to ensure the project would be finished within the time and cost limit, but also because the MOD took the wrong decision on what the platform should be for the new capability. In choosing the Nimrod (ex-Comet airliner), an old aircraft, they were directly responsible for many of the cost overruns. Yet any casual look back at the record reveals that other projects have also been subject to cost overruns and delays; a ongoing source of embarrassment as the MoD is constantly called to account by the Public Accounts Committee and the National Audit Office.

Getting the right equipment is of course vital, yet getting the right number of servicemen and women is going to challenge the armed forces in a way we haven’t seen before.

It is worth reminding him that in 2015, the Conservatives had a commitment to keep the army at least 82,000 personnel. Since then the Government has dropped this pledge and replaced it with plans to revitalise the regular reserve, although the target for that is I believe still too small.

Under Labour the reserves were allowed to wither away, even as the regulars declined as well. Yet this is very short sighted: if we aren’t going to increase the regular forces manpower we desperately need an increase in the reserves. After all, we saw time and again in Iraq and Afghanistan how so many of the regular units used reservists to ensure they weren’t below strength.

Another of Gavin Williamson’s headaches is that our perception of the threat we face is the key driver of expenditure.

A wide breadth of threats now trouble the UK’s intelligence and military chiefs. From the asymmetric global terrorist threat to outright conventional military action, as in the air strikes on ISIS in Syria and Iraq, there are many, often unforeseen, demands on the Forces.

Beyond that lurks the growing issue of Russian intentions, which could in turn drive calls for greater commitments than we have planned for, leading to overstretch.

To understand the nature of the growing threat posed by Russia, the Defence Secretary needs only look back over the last few years. Russia has invaded both Georgia and Crimea, been directly involved in the conflict in East Ukraine, and used Russian troops and aircraft in support of Assad in Syria.

All of this has been in pursuit of Putin’s goal of re-establishing Russia’s strong sphere of influence across the ex-Soviet states, and some of its neighbours who were once members of the Warsaw pact. Back in the Nineties no one saw this coming, yet it has now become a potential threat to the stability of the NATO alliance.

In the midst of this sits the need to re-visit the way that NATO works. My concern is that having spending targets doesn’t ensure that the quality of the spending is sufficient to create a common quality standard – and too many of the member states have consistently failed to meet this minimum objective.

With only United States, Greece, Britain, Estonia and Poland getting above the minimum required, and nations like France and Germany failing to even hit the target, it begins to make a mockery of it. As we push more member states to meet their obligations, perhaps we should ensure that the quality of their spending is also evaluated.

As NATO looks to control more operations, sometimes outside of the European theatre, they will need a more flexible approach to contributions. Some smaller countries could ensure, for example, that they had a wing of aircraft, fully modernised and inter-operable with NATO command, or perhaps elite troops. Then if NATO needs to deploy, it can quickly select the forces required from those who wish to be part of the operation.

That way the commitment wouldn’t just be money but as importantly functionality and capability. Whilst the UK and France with the USA would comply across all services, others would not. Thus a measure of quality would tell us much more about our likelihood of future success than simplistic GDP targets.

It is an enormous honour to be appointed Defence Secretary and I wish Williamson well. The list of issues is long but he is fortunate in the quality and dedication of our Armed Forces. The only advice I will give him is to remind the Chancellor and his other colleagues that the defence of the Realm is the first priority of our Government upon which all else rests, and for that to happen resources and commitments must align.