He retired as Chief of the Defence Staff, a post he had held for three years, in 2013. Before that, he had been Chief of the General Staff. Lord Richards saw plenty of action. He did three tours of Northern Ireland and commanded the 4th Armoured Brigade in Germany in the 1990s. He served in East Timor and Sierra Leone, where his initiative prevented revolutionaries from overthrowing the capital, Freetown, in 2000.

He then commanded Nato’s rapid reaction force, and the international force in Afghanistan, before becoming Commander-in-Chief, Land Forces. He brought a high level of practical soldiering to the CDS’s job, and it still underpins his thinking.

I ask him whether the world has become more dangerous since he retired. “I think it’s at least as dangerous, and potentially more. I give advice on geostrategy to governments around the world, and if I were to look at Britain, I’d say it’s a particularly dangerous time for a country that’s steering an unknown course post-Brexit and has global pretensions.”

North Korea is one such danger. “I think Kim [Jong-un] has no intention of provoking a war,” he says. “He knows it would be the end of him. But my worry is that he miscalculates, and America is forced to intervene. And the result of a military intervention will, in the short term, be awful. There should be a diplomatic outcome in which Russia – or, more likely, China – is the major influence.”

And what could Britain do? “Very little. This is not something we could any longer become involved in. Even if the new aircraft carrier were up and running, why would we send it all the way over there to add a few planes to the American effort? This is not one for us.

“For Britain, with its Army of 78,000 and its Navy of 20 frigates and destroyers, to have the conceit to think it can fight a war in the Far East is almost laughable. Our practical role should be confined to Nato, Africa and the Middle East. We lost all other capability not in the recent cuts, but in the cuts of the early 1990s, at the end of the Cold War.”

A year after Lord Richards ceased being CDS, he compared Britain’s declining capability to that of a “banana republic”. Now he says: “I exaggerated for effect. We’re certainly not at the banana republic stage. But the runes are not good.

“The nation’s apparent ambitions are not going to be met without putting significant amounts of fresh money into defence. At a time when we are leaving the European Union and there’s much talk of being global, and with a navy that effectively cannot get out more than 12 to 14 destroyers and frigates, even if you look just at the maritime component of military power, things don’t look good. It hardly fits an image of a prosperous Britain with active, vibrant, sizeable Armed Forces that can influence other nations.”

Some of the decisions that cut the Forces were taken on Lord Richards’s watch. “I do feel guilty in that I was part of a process that led to very disappointing outcomes. In 2010, when I took over as CDS, the decisions on the Strategic Defence and Security Review had been taken. Even David Cameron said it hadn’t been very sensible to let another CDS be the major influence on this.

“I should have owned the process. The way I rationalised it was that that country was in a hell of a state. We agreed to a 7.8 per cent cut, but were given all sorts of promises that, come 2015, things would improve: and I think, to be fair to David Cameron, he meant to deliver.

“But there was another cut the following year, and in 2015, while there was in theory a one per cent increase in equipment, overall there was a cut. So the Army today is far smaller than I signed up to as CGS, which I think was 94,000. My successor agreed to a cut to 82,000, and it’s only 78,000 today.”

A plan to use reservists to bolster the numbers has failed. “I’m afraid it hasn’t been able to deliver and never will, because of real combat capability. However proficient they are, a part-time soldier cannot be as effective as someone who’s devoted his life to it and puts on a uniform every day.”

Despite Kim’s instability, he says that “extremism in all its forms, notably jihadism, is the biggest threat. I don’t buy the idea that Russia need be the big threat everyone says it could be. There are regimes – and North Korea is obviously one – that could be pitched into doing things they perhaps don’t intend to. One day you can find yourselves fighting a war you didn’t expect, and I suspect we are neither psychologically nor physically ready for that to happen.”

I press him about Russia. “I think we’ve mishandled Russia since the end of the Cold War. I think good diplomacy, of which military activity is a part, and clear red lines will enable us to have a good relationship. We share many things in common, not least a concern about extremism. Russia doesn’t need to become a threat. In any case, we’re all dependent on America for any effective response to Russian aggression.”

The Libyan intervention also happened under Lord Richards: he says it was prompted by Downing Street saying that “we cannot have a Srebenica on our watch”, referring to massacre of Muslims in Bosnia in 1995. “Bearing in mind we had no plan for what came next, I had the temerity to say: ‘This is an opportunity to pause and negotiate with Gaddaffi.’

“But it had become a regime change operation, and there was a view in Paris and London that we were on a roll and ought to finish the job off. Asking what the plan was then was an inconvenient question. You shouldn’t go to war unless you have a good plan that you are confident in for the day after.”

He adds: “Even in 2011, Britain and France could not run a war against a pretty minor dictator, because of the technicalities of a modern military operation. We needed Nato, because we needed America. That should have been another lesson.”

Nor was Libya the only problem “Back in 2012, I gave the Government a plan to deal with Assad. There was no interest in it. We did enough to keep the war going, but not enough to give our putative allies there a chance to win.

“I said if our Government were not prepared to do this then it would be best to let Assad win and win quickly, because otherwise we were fomenting all sorts of other problems further downstream – but that was politically unacceptable. So we let it drag on. Then Russia intervened and demonstrated the decisive use of the military instrument. And now we are tacitly supporting Assad, because the real enemy is Isis.”

He claims that, then, Islamic State “was not a huge military challenge. They could have been dealt with in weeks. Now there are five to 10 million Syrians displaced, their lives ruined, hundreds of thousands killed. Much of that could have been avoided by early decisive action. But we couldn’t contribute the sort of force we had in 1990-91, or in 2003.

“If our ambition is to be the second military power within Nato, and to be conspicuously proficient so that the Americans see us as their partner of choice, then having a navy of 19 ships, 12 or 13 of which might be available, but even some of those prevented from getting out of harbour because of financial constraints, then we are in a pretty sorry state.”

He feels his own service is struggling. “The Army, which has made a gallant effort to retain all its skills, can’t do so because it can’t re-train soldiers to the highest degree. The Challenger tank becomes obsolete in four years. The Warrior vehicles are 1970s designs. There’s a lot of gesture strategy – putting 200 or 300 people in somewhere. We’ve got 350-500 people in Afghanistan now, but there’s a debate about whether we should do more, as America now is.

“It’s a prime example of how our forces can fight extremism. If Afghanistan collapsed tomorrow, because we haven’t finished the job properly, we’d have to start all over again and sort it out. We couldn’t leave Afghanistan as a massive rogue state exporting extremism around the region and to us.”

He believes the Army is desperately short of soldiers. “I think mass matters hugely. That’s why the numbers of ships in the Royal Navy and the number of planes in the RAF are so important, too. A ship can only be in one place at a time: and an example of where we are caught short through and absence of ships has been the hurricane in the West Indies. It’s not just fighting wars, extremism or insurgencies in which you need mass.”

Mass, of course, costs money. “Our ambitions and the requirements of the various challenges we are confronting cannot be matched by the capabilities we have.” The new aircraft carriers, which he opposed, “are having a huge distorting effect on the rest of the defence budget. Now we have them we have to make them work. But you make them work at the expense of the rest of the Navy, of the RAF and of the Army.”

He says we may have to revert to being “a maritime nation with a good little Navy, with the Army put on the back burner and used only in very selective ways. If we go on as we are, we won’t even deliver on the government’s goals for defence.”

Although retired, he is constantly among soldiers of all ranks – and discerns that morale is “fragile”.

“At the moment there’s a consensus that joining the Forces, whether you’re heir to the throne or the son of a dustman, is a good thing. If the government breaks that consensus by not looking after the people in the Armed Forces properly, word will get out. It will affect potential recruits and those we wish to retain.

“These people aren’t idiots. They know the sums don’t stack up. Recruitment and retention are very difficult. I think there’s a question about the government’s commitment, notwithstanding the much-vaunted military covenant, to people rather than equipment.”

He is concerned that married quarters aren’t being properly maintained, and there’s no commitment to their provision in the long term. The Major government sold them to the private sector, and from 2021, the owners can charge a commercial rent.

“It all creates doubt and worry, and so people take the chance to leave when they might previously have continued a career with the Armed Forces.”

He hopes an element of defence spending might soon be included in our overseas aid contribution of 0.7 per cent of GDP, to pay for more “mass” – and he knows ministers, including Priti Patel, the Overseas Development Secretary, share his frustration that it isn’t already.

“I think the Government must conduct a campaign with those who write the rules, and if that doesn’t work unilaterally extract themselves from the process. To not be able to include the military contribution to overseas aid is ridiculous.”

But then, he concludes: “There is an absence of grand strategic thinking in Whitehall. Where is Britain trying to position herself in the world in 20 or 30 years time? And where is the plan to get us there?”