The human cost of Britain's engagement in conflict in Iraq and Afghanistan, causing more than one front-line deployable soldier in five to be unfit, injured or channelled into administrative duties, was revealed by The Herald yesterday. Overstretch has taken an all-too-clear toll. Also yesterday, the Commons Defence Committee published a report disclosing the capital costs of military operations in both countries. The findings of this report are also a cause for concern. Fighting counter-insurgency campaigns does not come cheaply and no-one should begrudge the investment needed to equip, protect, pay and support British troops who put their lives on the line.

What is worrying is the way predicted costs for Britain's involvement in Iraq have spiralled in the past three months. When the Ministry of Defence (MoD) winter supplementary estimate for 2007-8 was presented in November, the cost of operations in Iraq was put at £955m. For Afghanistan, the estimate was £964m. According to the latest, spring, MoD estimate, the Iraq bill has increased to £1449m, rising to £1424m for Afghanistan. The forecast for Afghanistan is perhaps not surprising, given the increase in British troop numbers in Helmand and the nature and intensity of the fighting, which might well escalate in the spring.

In Iraq, however, the number of troops has fallen, to 4100, and the government is committed to a drawdown to about 2500 from spring this year. In addition, British forces are based at Basra Air Station, where they have taken on an "overwatch" role after the handover of day-to-day, front-line operations to Iraqi security forces. It is not only members of the defence committee who are curious about a situation in which troop numbers come down, but operational costs go up. The figures will cause the taxpayer, while wishing to ensure that British forces are given the resources to do their job and stay as safe as possible, to reflect anew on the human and financial costs of invading Iraq, five years on from an event that has had baleful consequences for that country and beyond.

How did the MoD get its estimates so wrong for the financial year soon ending? The committee wonders whether the nature of operations in Iraq in the past few months threw otherwise robust forecasts off course, or whether November's estimate was especially weak.

There is little to suggest it was the former. If the latter, it would reflect very badly on the MoD. Did the difficulty of trying to provide robust forecasts discourage the MoD from providing a thorough forecast? It needs leeway to meet unforeseen costs in an unpredictable environment. But so much financial leeway? On such obfuscating terms? The MoD correctly has been left in little doubt that the committee expects a full explanation. So does the public. Costing conflict is an imprecise science. The difficulty in providing reliable estimates is no excuse for the MoD engaging in a fog of war of its own.