A rather dull election campaign has suddenly gone nuclear, with the main parties going head to head over the replacement of the Trident weapons system amid accusations that Britain's defences are being compromised as a matter of political expediency. But Trident is only one weapon in our armoury, though admittedly it costs a bomb and many would argue that we don't actually need it. The defence of the realm is the first duty of government and it is arguably not nearly as high up the agenda as it ought to be.

Armies, navies and air forces are very expensive items, and at a time of severe financial pressure finding money to fund them is a tough sell. Added to this is the public perception that we have not made very good use of our military forces in recent years, and it gets harder. For Christians, the question is sometimes complicated by the whole issue about whether warfare is justifiable anyway.

On the other hand, it's a dangerous world, and not many people would argue that we can do without a military altogether (though there are a few). Whether we need it to be as large or expensive is another question. So should we spend lots of money on our armed forces, or should we spend it on hospitals and schools?

So what's all the row about Trident?

It is a nuclear weapons system deployed from four submarines, at least one of which is always at sea and theoretically able to avoid detection and pre-emptive destruction. These are coming to the end of their lives and the big argument is over whether they should be replaced.

Sounds expensive.

Yes. The government estimated around £20 billion in 2013, though the record of successive governments in controlling costs is so poor that it's anyone's guess.

That would build a lot of hospitals. Do we really need it?

Good question. Nuclear weapons would not be used in an offensive war, but serve only to deter attacks from another nuclear power. Whether this is likely is an open question, and many would say that it isn't. On the other hand, the possibility of a nuclear attack is so dreadful that supporters argue that all possible precautions should be taken against it. On that basis, it's argued that the strategy of Mutually Assured Destruction has actually worked, since there has been no nuclear war, and we would be extremely foolish to abandon it.

Others argue that we don't need an independent deterrent because the Americans have lots of nukes and they would defend us. This might be regarded as unethical, in that we're letting other people pay for our defence, and rather optimistic, in that we are expecting the Americans to run the risk of a domestic nuclear holocaust for the sake of us Europeans.

But nukes – they're just wrong, aren't they?

Sure. Some people – many Christians among them – argue that Britain should not on principle have weapons like Trident because of the damage they do. On the other hand, 'advances' in military technology have always been morally questionable: the famous Chevalier Bayard in the 16th century used to have captured crossbowmen executed out of hand because their weapon was unsporting. But what really matters is not whether we have nuclear weapons, but whether we have a nuclear war. The doctrine of 'mutually assured destruction' is deeply unpalatable, but if a balance of terror is needed to stop countries using their nukes it would arguably be unethical to dispense with them. The calculation depends on all sorts of complex strategic, tactical and geopolitical analyses. We can all agree that the world would be better with no nuclear weapons at all; the question is how to get there. There are some strong arguments for saying Britain does not need an independent deterrent, but just saying "nuclear weapons are bad" is really not one of them.

What do the political parties say?

The Scottish Nationalists are against them on principle and the Conservatives Defence Secretary Michael Fallon has said that if they go into coalition with Labour they will pressurise Mr Miliband to scrap them. This has been categorically denied by Labour, which says they will explore cutting the number to three submarines but haven't decided yet; the Lib Dems think the same. It is generally thought that four are needed to preserve a continuous at-sea presence. The Conservatives have announced that they'll commit to four in their manifesto.

You said that Trident was only one issue.

Yes. Another is how much we spend overall, and according to President Obama, we are not spending enough. NATO's target for its members' defence spending is two per cent. At the moment Britain is just over that, but it is set to fall to just over 1.8 per cent by 2017. However: behind these numbers lie bigger questions about what our military is for, how it is used and what Britain's role in the world ought to be.

Rule, Britannia!

You are a little out of date. We no longer engage in wars of conquest. Our military force is used to uphold international law and defend Britain's interests. It's also the last line of defence in case of civil unrest. The Coalition goverment reduced the army to only 82,000 troops – by no means all of them front-line soldiers – and this is set to fall to only 50,000 by 2019, making it the smallest it has been since the American War of Independence.

On the face of it, that's a bit worrying. The world is a dangerous place, after all.

A string of senior military personnel have warned that Britain might not be able to do all it needs with the few troops it will have at its disposal. The rise of Islamist movements in the Middle East and Africa and a resurgent and territorially aggressive Russia are deeply worrying. Many people would argue that this is not a time to be cutting the size of our armed forces.

I see that, but on the other hand all these things are happening a long way away. What have they got to do with us?

A good question. Some people – like the Greens, for instance – argue that since we're never going to be invaded, we don't really need an army at all. But we also have an army, as the Defence Vision puts it, "To defend the United Kingdom and its interests, strengthen international peace and stability, and act as a force for good in the world." To do that you need to be able to project a credible force wherever it is needed.

What, like Afghanistan and Iraq? They didn't go so well, did they?

The perceived errors in the way those two situations were handled have made what is deemed to be military adventurism deeply unpopular, and this is having an effect on the debate. However, they are very different. Afghanistan was a failed state whose violence and corruption was destabilising surrounding nations. If you have a strong stomach, read Ahmed Rashid's book Taliban for a picture of the country and surrounding region before the 9/11 attacks. Atrocities too gruesome for a family website were routine. Opponents of the intervention – who rightly point out the cost, the high civilian death rates and the many mistakes that were made – have no responsible alternative, and Afghanistan today is in a better place than before the invasion.

Iraq is a harder case, and a text-book example of a country – the US – which knew how to win a war but not how to win a peace. However, we do not know how Saddam's regime would have ended without Coalition intervention; Libya didn't turn out well either. Nigel Biggar in his important book In Defence of War argues that Iraq was justified.

I can't say I'm convinced.

Fair enough, and these are not knock-down arguments. However, military intervention in Bosnia is widely held to have saved lives and brought a miserable war to a close. If the international community had not been so squeamish, this might have been done much sooner. Britain's unilateral intervention in Sierra Leone brought a truly vile rebel group to its knees (they specialised in amputating children's hands) and is warmly remembered there. Military intervention in Rwanda might have saved hundreds of thousands of lives. French troops are crucial in helping Mali fight off its extremists. My point is that sometimes it's necessary.

Alright, but why does it have to be us?

It doesn't. Many countries have large economies but inefficient or under-funded armies. Historically, though, we have developed a first-class military with very good tactical and strategic abilities. One answer, then, is "because we can". If we decide that we won't do war at all, or only on a limited scale, that leaves it to others who might not have the same capabilities or moral considerations as our armed forces. Another is that if you believe wars sometimes have to be fought, it's not very ethical to leave it all to someone else – ie the Americans – to do it for you. Another is that without a stick of some size to back up soft speech, no one will listen.

So the whole pacifist we-should-all-sit-down-and-talk-about-it thing doesn't really work for you?

In the sense that war should always be a last resort and that much can be achieved through negotiation, yes. As a statement of ultimate principle, no. It founders on the fact that not everyone can be reasoned with. Some people and some regimes have to stopped by force, or the credible threat of force, or they won't be stopped at all. It is one of life's ironies that the freedom not to believe in war is secured by the efforts of large numbers of people skilled in the use of deadly force. Armies also need an efficient and innovative arms industry, another frequent target of critics.

Do the Churches have anything to say about this sort of thing?

Not often much that is very useful. Churches are good at pastoral issues, eg chaplaincy, but have an institutional resistance to appearing militaristic which sometimes means that their contributions lack rigour. Charles Reed in Just War (2004) analysed various Church responses to war and concluded that "in all instances the British churches appear to place in the public domain arguments and recommendations that politicians can rarely, if ever, implement". He thought that Church thinking was dominated by Cold War thinking while the world had become a fundamentally different place.

So whom should we vote for?

The economic situation has left both major parties in difficulty. Even to maintain defence spending at current levels would need an increase of £8 billion over the next parliament, and David Cameron has declined to commit to that. Ed Miliband has also refused to commit to meeting NATO's two per cent commitment, but has promised an immediate defence review if he is elected. In principle that might mean spending goes up, though it is unlikely. The Liberal Democrats would scale back the nuclear deterrent rather than abolishing it.

It's all very difficult. I suppose defence just isn't a vote-winner.

Well, there are signs it is moving up the agenda, but it is indeed difficult. It is tempting to think we could just spend all the money we use on the armed forces on schools and hospitals and everything would just go on as before. However, the world would probably be a more dangerous place, and not just for us. So we need a strong military; how strong it's very hard to say, but the first responsibility of any government is the security of its people. Defence really matters.