The Trident nuclear deterrent is the one issue that divides even the ruling Labour Party in Scotland, polarising the camps of those implacably opposed to atomic weapons and those willing to support the government line of "a prudent insurance policy in a dangerous world".

Based at Faslane on the Clyde, the four Vanguard-class submarines that carry the missiles complicate the issue by providing work for 7100 local civilians and another 4000 sub-contractors across the central belt. The 100-acre site is the single largest industrial employer in Scotland, contributing £180m a year to the local economy through contracts for everything from catering to construction, and £280m to the wider wealth north of the border.

Politicians of all parties for whom nuclear arms are anathema talk vaguely of "alternative" jobs if Faslane and the neighbouring armaments depot at Coulport are ever closed, but have few realistic ideas of what would replace them as the economic linchpin of an area where non-defence-related work is scarce and opportunities limited.

The other side of the coin is playing host to enough nuclear "throw-weight" to devastate a small continent. Each Vanguard that slips its moorings and heads off into the Atlantic on round-the-clock deterrent patrol carries 148 warheads, each eight times as powerful as the crude bombs that destroyed Hiroshima and Nagasaki in 1945.

The weapons can be launched from a range of 5000 miles and are accurate to within a few dozen yards. The improved US missiles being considered to equip Britain's next generation of submarines - also scheduled to be based at Faslane - will be able to hit their targets with pinpoint accuracy to counter underground command bunkers and nuclear storage sites.

The nuclear dimension apart, defence plays an understated but central role in Scotland's economic life, from the shipyards of the Clyde to the high-tech factories around Edinburgh and Dunfermline designing laser-guidance pods and advanced optics for military jets. Despite the drastic 1980s cull of "metal-bashing" firms and the collapse of the steel industry, the Govan and Scotstoun yards now owned by BAE Systems have survived to take pole position in UK warship-building hierarchy.

Between them, they employ 3300 skilled workers and indirectly support a further 1200. The yards are currently engaged in construction of the second pair of six Type 45 air-defence destroyers for the Royal Navy. Two of the advanced ships have already been launched.

Soon the yards expect to be starting the biggest contract in their history: to build key parts of two 60,000-tonne aircraft carriers due to come into service in the latter half of the next decade. That £3.8bn deal, on course to be signed this summer, will provide work for all of Britain's surviving warship yards. The Clyde, along with VT in Portsmouth, will build modular sections of the huge ships. The component parts will then be towed to the east coast to be welded together at Babcock's facilities at Rosyth.

The University of Strathclyde's Fraser of Allander Institute says that for every 100 direct BAE jobs on Clydeside, company contracts underpin another 57 in logistical back-up industries from electronics to forestry. Last year, Govan and Scotstoun contributed £238.2m to Scottish coffers, including £107.7m in wages. Vic Emery, BAE's managing director, says: "The economic numbers are significant. The shipbuilding industry is at the very heart of the Scottish economy."

Babcock Engineering Services, BAE's nearest rival, has 1200 workers at its east-coast site and specialises in refitting and upgrading major naval vessels. Its naval division also runs Faslane's site infrastructure. In 2006 it completed a £25m, two-year makeover of the 22,000-tonne carrier and assault ship Ark Royal, still the navy's de facto flagship.

There are about 170 companies in Scotland involved in defence in the aerospace and marine sectors. Between them, they employ 15,000 people, and in 2005, they generated more than £2.1bn of business between them. Employees in the crucial electronic technologies at the core of the Ministry of Defence's future procurement strategies also earn one-third more than the Scottish average wage.

The companies are investing for the long term, too: the latest figures available show more than £91m spent on research and development, and the hiring of 500 apprentices. On average, the MoD spends between £600m and £700m a year sustaining defence manufacturing jobs north of the border.

Lieutenant-Colonel Stuart Crawford, a former tank commander and now a military analyst, says Scotland has always punched above its weight in military and industrial terms. "We have about 8% of the UK's overall defence manufacturing jobs, and make up almost 10% of the total manpower of the three armed services and 13% of the army from a population which represents 8.7% of the total British population," he says. "The MoD employs 6660 full-time and 440 part-time civil servants up here. Glasgow is the headquarters of the Army Personnel Centre, the site from which every soldier's pay, postings and career are managed worldwide.

"While recruitment has suffered in recent years due to factors such as regimental amalgamations, Iraq, better educational opportunities and a general demographic decline in people of military age, the five Scottish infantry battalions still make up more than 12% of the army's available ground troops."

There are between 19,000 and 20,000 Scots in the regular army, navy and RAF, with a further 2500 in the part-time Territorial Army. Hundreds of the latter have already served six-month stints on the frontlines in Iraq and Afghanistan.

The army headquarters near Edinburgh is responsible for 19,000 personnel scattered from the Shetlands to South Yorkshire, and three of the RAF's operational bases are located in Scotland. Lossiemouth and Kinloss in Moray and Leuchars in Fife are frontline flying stations; between them, they operate Tornado strike and fighter jets, Nimrod maritime patrol and intelligence-gathering aircraft, and search and rescue helicopters.

The Sea King helicopters from the rescue centre at Kinloss saved 1700 lives in 2006, and its crews were scrambled more than 3000 times for missions all over the UK. These included airlifting stranded or injured climbers and winching crewmen from sinking boats around the coast.

The RAF has almost 6000 uniformed and more than 1000 civilian employees in Scotland, many of whom live "off-base" in surrounding communities and contribute directly to the local and national economies.

The MoD owns land and buildings covering 25,000 hectares and thought to be worth £1.3bn. There are also Nato warship refuelling depots at Campbeltown, Loch Striven and Loch Ewe.

The Royal Marines have one commando group of 700-plus men based at HMS Condor on the outskirts of Arbroath, and a second 530-strong Fleet Protection Group stationed on the Faslane site. The latter unit provides the specially trained counter-terrorist manpower to protect the North Sea oil and gas rigs and platforms off Scotland's coasts; it also supplies anti-ambush teams to escort the road convoys transporting refurbished warheads from the government laboratories at Burghfield in rural Berkshire to Coulport.

Like their RAF counterparts, many of these servicemen and women elect to buy or rent property in the Helensburgh/Rhu area and become part of the wider civilian community. There is also a healthy population of "galvanised Scots" - former naval officers and senior ranks from elsewhere in the UK who choose to retire to homes near the base.

Trident or no Trident, the future of Scotland's defence industry is a vitally important issue.

"The figures are overplayed"

Nestling between the naval base at Faslane and the Coulport depot is a lush garden described as a "flicker of light within the shadow of darkness". This is how Dr Jim Taggart, a retired botanist and veteran opposer of the UK's nuclear missile programme, sees his home at Linn public gardens on the Rosneath peninsula.

Dr Taggart, 72, who was raised in the Argyll and Bute area, says: "Governments have an obligation to protect civilians. There is no way nuclear weapons can be detonated and still comply with these principles."

George Freeman, independent councillor for the area, insists the majority of local people are in favour of the Faslane base. "There is a fair amount of support locally for it. Try taking 11,000 jobs out of any local economy. The area would be devastated," he says.

But Dr Taggart is sceptical. "People say these bases bring jobs to the area, which I don't dispute, but the number of jobs brought here is grossly overplayed," he says.

Dr Taggart has called instead for a "programme of conversion to sustainable non-nuclear jobs" such as those in renewable energy, conservation and sustainable forms of public transport.

Meanwhile, Hoosie, a peace activist who has campaigned at Faslane for around 10 years, says the money spent on maintaining the Trident missile programme in the UK would be better directed towards "essential" community services.

"Decommissioning the bases would require more workers than it takes to maintain them. And the £2000 a minute it costs to maintain our nuclear programme could be redirected into facilities such as schools and hospitals."

What the parties say

Labour

Supports the renewal of Trident, and argues Westminster should continue to decide on defence matters. Stresses the benefits to Scotland of being part of the British armed forces. Promises to continue to work with Whitehall to win defence contracts and boost the Scottish economy.

SNP

Wants Scotland to make its own decisions about defence. Says it will ensure that Scottish soldiers are never again dragged into an illegal war. Wants to maintain active defence commitments through the UN, European Union and Partnership for Peace. Opposed to Trident and nuclear weapons.

Conservative

Believes in retaining independent nuclear deterrent. Condemns any further cuts in armed services and wants to encourage recruitment. As a matter of urgency, would address the current level of overstretch in the armed forces.

Libdem

Has a commitment to work for the elimination of nuclear weapons, but would retain the minimum nuclear deterrent until progress has been made to this end. Would continue to improve the capability of the army so it can face the strategic challenges of modern-day conflict.

Green

Wants to scrap Trident. Would make transportation of nuclear warheads on Scottish roads and inshore waters illegal, and wants UK soldiers to be withdrawn from Iraq and replaced with peace-keeping forces from countries not involved in original invasion. Training of forces should prioritise peacekeeping.

SSP

Against Trident renewal and wants the subs removed from the Clyde. Full support would be given to current members of the armed forces. Guarantees re-employment or re-training for existing members, who would not be displaced.

Solidarity

Wants the elections to be a referendum on Trident, and is recommending electors to only vote for candidates opposed to Trident. Says an independent socialist Scotland would be a force for peace and justice internationally.