With our long engagement in Afghanistan moving into its final chapter, we must now debate the role of our armed forces beyond 2015, when the next defence review is due. In my view, despite successive reviews forcing change, we are still too configured for state-on-state warfare.

On Monday, in the light of events in Mali and Algeria, David Cameron rightly suggested that the threat from international terrorism will have to be contested for decades to come. It is a threat we will have to tackle in co-operation with others, not on our own. And it is a threat against which we must wield strategic patience, avoiding the impulsive and costly errors of the past decade. Much visionary thinking has been undertaken by UK military chiefs about the Future Character of Conflict and the lighter, nimbler, more flexible and adaptable forces we will need. Getting our people into potential trouble areas before problems arise and political engagement will be vital. One of the most uplifting things I saw as defence minister was a training camp in the Sierra Leone bush, where we were helping train a battalion for an Amisom peacekeeping mission. Amazing when you think where Sierra Leone was just a decade ago. Sadly, we are currently cutting our effort there.

By cruel irony, the day after Cameron's statement, ministers were back in the Commons defending the latest army redundancies. Are we cutting the right things? Central to our narrative while making the cuts was the assertion – valid in my view – that if our predecessors has faced up to difficult decisions earlier, we would not have had to take such desperate measures.

But defence ministers in post later this decade will face even grimmer choices, and make that case even more forcefully, if big decisions are not made by 2015. Central among these will be the question of whether and how to replace our Trident nuclear deterrent.

In the decade or so from 2017-18, the current plan is to spend between £25bn and £30bn building four vast new submarines whose sole purpose will be to patrol the high seas 24/7 waggling our nuclear bomb at – er – no one in particular. For another 30 years we'll spend £3bn a year in today's money operating them, and one day it will cost several billion more to decommission our nukes. Yet our national security strategy has downgraded the nuclear threat to "secondary", and we have had no identified nuclear adversary since the end of the cold war.

But in the same decade, we will have to pay for: the F35 plane to fly off the new carriers; Type 26 frigates; whatever remotely piloted aircraft we end up building; and whatever amphibious shipping is to replace HMS Ocean and her like. The army must eventually be bought some kit fit at least for the 20th century, even if the 21st is a bit hopeful, and we must urgently expand our Istar – intelligence, surveillance, target acquisition and reconnaissance – capability. You cannot do all of that on the existing size of defence budget, even assuming that it can be conserved at today's value in real terms (a big if!). In fact, you probably can't do quite a lot of it. So it is absolutely essential that any further generation of nuclear deterrent must take its place alongside all those other items – far more relevant to the action we really will be involved in – on the table for debate.

If we give Trident an automatic bye we will become even less capable of protecting our real interests through joining in international action to tackle and pre-empt real threats. If replacing Trident like for like on its cold war scale comes at the expense of the rest of the Royal Navy's capabilities, this would have a devastating impact on our global reach. We already ask the navy to do too much with too little, as far apart as the south Atlantic and the Gulf. Cuts in the frigate programme would greatly reduce what can be done even further. Failure to complete the carrier-strike project would risk leaving us with two white elephants.

And failure to replace HMS Ocean and the other amphibious shipping we have lost with a new 21st century generation of such assets would drastically curtail the work of the Royal Marines and our ability both to land forces from the sea, and sometimes evacuate people by sea. This will be the bread and butter work of the armed forces; we will need to fulfil the more expeditionary role being envisaged for them.

In the air we need more helicopters, as recent reports have pointed to at least a decade of shortfall in rotary capability – again vital both to our reach and to humanitarian work in tricky terrain. We have thin coverage of air surveillance assets and need far more to equip us for the long battles against insurgents that the PM described. More investment in the cyber domain will also be needed.

And on the land, in various parts of the globe, we need professional and properly equipped forces capable of deploying quickly and flexibly. Current plans see numbers cut, but as yet no corresponding upswing on equipment.

Sacrificing any of these for the sake of sustaining our nuclear capability at 1980 levels puts us at risk, and means that our perception of ourselves as a significant force at the world's top table will be based on historical anachronism rather than current capability.