It is a fundamental job of any government - keeping its people safe.

But how best to do that has already emerged as one of the key battlegrounds ahead of May 7.

One of the most prominent issues - forced up the agenda by a surge in SNP support - is Trident.

A decision over renewal of Britain's nuclear deterrent will have to be made in 2016, meaning what the next Government decides to do will have to implications for defence for decades to come.

Supporters see an independent nuclear capability as the ultimate deterrent against attack, and argue scrapping it would be foolish in an uncertain world, with Putin's Russia looking increasingly threatening, and regimes in North Korea and Iran potentially gaining the capability to launch a nuclear strike against Britain in the years ahead. It also gives the UK gravitas as a major world player, and it has been claimed that the UK could be marginalised on the UN Security Council, where it retains its permanent veto, without Trident.

Others, however, see Trident as a moral obscenity. The four submarines - one of which is constantly at sea - are designed to unleash untold, indiscriminate destruction, wiping out cities, killing millions and inflicting catastrophic environmental damage at the push of a button. It is a tool, it is argued, that can never be used by a civilised state.

Others point to the practical reasons for scrapping Trident, particularly at a time of cuts on public services. The cost of replacing Trident is disputed, but will certainly be substantial. The Ministry of Defence has said it will cost £17.5bn to £23.4bn to procure the replacement system, while the Campaign for Nuclear Disarmament has estimated total costs as £100 billion. Would the cash be better spent on schools and hospitals? It has also been slammed as a Cold War relic, useless against the stateless terrorist groups or cyber crime which are becoming increasing threats.

The three main Westminster parties are agreed, officially at least, that Trident should be renewed. However, Labour has not ruled out cutting the number of submarines to three, while the LibDems back a reduction arguing it is not necessary to have a nuclear-armed submarine at sea constantly.

The issue of defence spending has also reared its head. NATO sets a non-binding target that two per cent of GDP should be spent on defence, and David Cameron encouraged his allies to meet the goal only last year at a summit in Wales. However, he has since failed to guarantee that his own party would meet the target if reelected. Generals fear that Britain's status in the world and military strength would decline further if defence budgets continue to be slashed.

CONSERVATIVES

*The party strongly supports the renewal of Trident. David Cameron has argued that Britain would be foolish to scrap what he calls "the ultimate weapon of defence." The party has sought to play on fears among some voters that the SNP could force Ed Miliband to scrap Trident in exchange for support that would make him Prime Minister.

*The Tories are not likely to commit to spending two per cent of GDP on defence, in line with the NATO target, having presided over heavy reductions in spending in recent years. The position has angered some on the backbenches who believe it is vital Britain remains a global power and defence chiefs.

*David Cameron has said he won't allow further reductions in the regular army, which is due to fall to 82,000 by the end of the decade - down 20,000 on 2010 figures. He has also pledged to increase spending on military equipment.

LABOUR

*The leadership supports the renewal of Trident, although plenty of left wing backbenchers and activists do not support the position. The official policy remains to replace all four submarines, although it is also open to exploring whether three would do the job.

*Ed Miliband does not come across with all voters as a natural military leader, but it is an image he has been keen to dispel, pointing out that he voted for action in Libya. "I'm not a pacifist," he told Jeremy Paxman... "Hell yes I'm tough enough."

*Labour supports a strategic defence review, which would look specifically at the threat of cyber-warfare. It is likely to make further reductions to the budget, and favours slashing top brass.

LIBERAL DEMOCRATS

*The LibDems think Trident is an "out-dated and expensive Cold War relic." But they don't want to scrap it. Instead, they would reduce the number of submarines and there would not be one on patrol constantly. They could be deployed if the threat from a nuclear-armed state rose.

*Again, there has been no commitment to keeping defence spending at two per cent.

*Dr Julie Smith, chair of a recent Lib Dem Defence policy paper, said last year that it would be sensible to pool and share equipment with allies in the EU and NATO.

SNP

*The party is strongly opposed to Trident, arguing that the renewal of Britain's nuclear deterrent is a "red line issue" in a possible deal with Labour. However, Nicola Sturgeon has softened her position slightly recently, refusing to say whether she would bring down a Labour Government if it forced through renewal with the help of Tory votes.

*The SNP remains committed to NATO membership, after the party agreed to reverse long-standing opposition to the nuclear alliance in the run-up to the referendum. The decision remains unpopular with many of its members.

*The party believes that the Faslane base, where the nuclear deterrent is currently based, should become a conventional naval base.

SCOTTISH GREENS

*The Scottish Greens believe the time has come for a radical reappraisal of the role the UK plays in the world. The party's manifesto states that a Green Government would deliver "a foreign policy based on peace, a global deal on climate change and a halt to the arms trade."

*The party strongly opposes the nuclear weapons, and would like to see Trident nuclear missiles disarmed and Vanguard-class submarines scrapped. Workers would be reskilled and redeployed to provide defence of the strategically important northern seas from Faslane and Coulport.

*The Greens would leave NATO and reduce military spending, with the armed forces primarily taking part in UN-sanctioned peace keeping missions, conflict resolution and playing a role in humanitarian efforts internationally.

UKIP

*UKIP has sought to present itself as the party of defence, pledging to meet the two per cent NATO target. Nigel Farage has said it would will plough a cumulative £16bn extra into defence during the course of the next parliament compared to the other parties.

*The party has vowed to ensure veterans have priority access to mental health services, social housing and jobs with the police, border force and prisons.

*It would create a dedicated minister for veterans, attached to the Cabinet Office, not the MoD.

DUP

* Jeffrey Donaldson, DUP defence spokesman, has said that maintaining two per cent of GDP on defence would be a key demand in talks if its support is required to form a Government. "It's not just a NATO requirement, we need this for our defence," he said.

* The party is against scrapping Trident, with its MPs voting against a motion calling for its non-renewal in January. South Belfast DUP candidate Jonathan Bell has said the nuclear deterrent should not be used as a bartering chip with the SNP. He added: "Both the Conservative and Labour Parties have stated their support for replacing our nuclear deterrent. Such policies must be taken on the merits of the case, not on whether SNP votes might be needed to form a government."