Greg Rosen: History holds a sobering lesson for Lib Dems

In truth, the idea of a coalition government is less radical than some London commentators might have us believe. In some ways, their shocked tones reflect a simple lack of awareness of the fact the formation of the Con-Lib coalition could be seen as the Westminster parliament dipping its toe in the water of a "new politics" pioneered in Scotland by Jim Wallace and the late Donald Dewar some years ago.

More fundamentally, it fails to acknowledge that two of the most successful and effective governments of the 20th century were the coalitions led by Lloyd George during the First World War and Winston Churchill during the Second World War. The participation of the Liberal Party in the Churchill coalition was, in fact, the last time Liberal MPs served in government. Many Liberal Democrats today will be able to point to that government as a model for this: not least Lib Dem MP John Thurso, whose grandfather, Sir Archibald Sinclair, was the Liberal leader and the most senior Liberal minister in it.

Such is the nature of political honeymoons that it will be tempting for many in the London press pack to paint Clegg and Cameron as a reprise of the Churchill coalition of 1940-45. Their focus on areas of common ground will be vital to the success of their administration, for the enduring insight of Disraeli's attack on coalitions is that voters lose patience with a government that lacks coherent purpose or is so divided in itself that its ability to resolve public policy issues is undermined. It is a lack of shared common purpose that can turn a government of "all the big beasts" into a bear pit of political egos.

Coalitions can be just as disastrous, not just for the country, but for the parties involved. It is far less likely that John Thurso will want to be reminded of the fact that the role of the Liberals in the wartime coalition earned not the gratitude of the electorate, but the defeat of his grandfather, at the 1945 election. In their defence, the Liberal Party was already in a fragile electoral state by 1945, when its 20 MPs were reduced to 12. And it is the causes of that fragility which represent the real lesson for Clegg and his party today.

The parlous state of the 1940s Liberal Party was directly attributable to its collective mishandling of the hung parliament elected in 1929 and of the coalition government it joined in 1931. As now, the 59 Liberal MPs elected in 1929 were not prepared to do a formal deal with Labour. Their leader, Lloyd George, would have wanted to, but too many Liberal MPs said that they would not sign up to such a deal, making it impossible for Lloyd George and Labour leader Ramsay MacDonald to make the numbers add up. It was this that forced MacDonald to lead a brittle minority government and ultimately was to lead to its fall.

What followed was a "National government" led by MacDonald, including a smattering of former Labour ministers, and most Liberals. Despite the fact the overwhelming bulk of the MPs supporting the National government were Conservative, Tory leader Stanley Baldwin cleverly allowed MacDonald and leading Liberal figures to take a significant number of ministerial posts. It was a strategy that bound them into a government whose central agreed objective was a "cuts" agenda – ostensibly to prop up the UK exchange rate, which at the time linked the pound to the price of gold. Central to the deal was an agreement not to fight an early election as a coalition, but to disband the coalition once the immediate crisis had been resolved. In the event, the Liberals allowed themselves to be trapped by the temptation of ministerial jobs. Once in government, they found that Conservative pressure within the coalition led to gradual evolution of the terms of the deal.

At each stage, the change was too small to precipitate the resignation of the Liberal ministers. The first mistake the Liberals made was to allow themselves to be talked into fighting an early election on a joint "coalition" platform with the Conservatives. While protecting Liberal seats, it made them dependent on "borrowed" Conservative votes. Having bound the Liberals to themselves electorally, on the suggestion of the then Lord Hailsham (who, like Edmund Blackadder, seems to have an incarnation in most eras of our history), an "agreement to differ" enabled Liberal ministers to vote against their own government's policies while remaining in office. The government then implemented the policies which in some cases the Liberals had come into politics to oppose. The cumulative ratchet effect meant that by late 1932, Liberal ministers were passengers in a government committed to very different ideals than those of their supporters.

Eventually, faced with embracing economic policies they could not stomach, half the Liberals resigned, including John Thurso's grandfather, whose role as secretary of state for Scotland finds echo today in the role of Lib Dem MP Danny Alexander. But half stayed, arguing the issue was not sufficiently important to jeopardise their coalition with the Conservatives. Liberal voters and activists were bewildered, and deserted their party in droves. By the time Sir Archibald led his Liberal team into Churchill's wartime coalition, it was a pale shadow of the once-great Liberal Party of old.

By insisting on a fixed-term agreement, Clegg is wisely guarding against the risk Cameron might call an election at a time when Tories are far more popular and Lib Dems are not – the fate that befell the Liberals in 1931. But he will need to do more to protect his party. By giving Lib Dem MPs Vince Cable and David Laws key economic portfolios, Cameron has cleverly tied the fortunes of the popular Cable to those of the less popular Osborne, and positioned the Conservative Party to be able to lay the blame for less popular aspects of the "cuts" agenda on the Lib Dems. Lib Dem members and voters will be hoping Cable, Laws and others will use their influence to moderate Tory policies. No doubt they will try. But if they can't, they will have to admit the failure of their participation in a Conservative-led coalition. It is hard – embarrassing even – for any politician to admit failure. For the Lib Dems, it would be worse. It could destroy their party for a generation.