Unity is a precious commodity in politics. In the modern era, when the media will pounce on the smallest schisms and polls repeatedly show the public strongly in favour of unity, parties are loathe to give breathing room to policy disagreements as once they might have. Keeping up the appearance of a united front is very important to today's spin doctors. But it tends to be a rule that the more a party needs to talk or think about unity, the less united it is.

But when you get beneath the cosmetics of media management, true unity is critical to great political success, a fact which helps to explain why both of Britain's major parties are having a hard time of it at the moment.

I recently saw something which perfectly summed up my point. HBO's smash-hit television show Game of Thrones kicked off its third season this week and in preparation I've whiled away much of my free time catching up on the first two seasons. Since the show is all about Machiavellian political intrigue there are plenty of parallels that a political commentator might recognise, and one in particular leapt out at me.

In this scene (starting two minutes in), two rulers debate the strategic consequences of a foreign invasion. The queen moots that all will be well, since their forces greatly outnumber the enemy's. In response, if you're disinclined to watch the clip, the king asks which is the greater number, 5 or 1. The queen says 5, at which point he holds up an open hand and a clenched fist. Gesturing with the fist, he explains: "One. One army, a real army, united behind one leader, with one purpose", can overcome a much larger but ill-organised enemy.

The history of warfare is full of examples of a small, cohesive and disciplined force overcoming superior numbers. Such a force can be counted upon to understand and execute orders, move with celerity, and be brought to bear in concentrated strength on a point of enemy weakness. The leader, not being distracted by keeping their army together, can focus on the opposition.

In politics as in war, genuine unity is an incomparable source of strength. Postwar British politics provides two examples of this sort of unity. Lady Thatcher isolated the Wets and, her right flank secure, broke the Conservatives out of the traditional voter base and into new electoral territory. Whilst Thatcher was uniting the right, the political left was fracturing: Labour was fighting for its own soul against far-left infiltrators whilst shedding social democrats to the SDP. The left was too busy fighting over what it meant to be "left" than it was fighting the right, and Thatcher was able to exploit their various weaknesses ruthlessly.

After she fell, the Conservative Party lost cohesion and proceeded to disembowel itself over the course of the Major government. Now it was the turn of the British left, united under Tony Blair, to break deep into traditional Conservative territory and wring landslide after landslide from a right that was more preoccupied with mourning Thatcherism than updating it.

What both Thatcher and Blair have in common is that they didn't pursue unity by trying to please everybody. Instead, each built a winning coalition and overawed the doubters. Blair swept the Labour left along in his wake, whilst with the exception of a single defector to the SDP the Tory 'left' remained within the Conservative tent under Thatcher. Surveying the present political landscape, it is obvious that neither Labour nor the Conservatives have achieved this happy state today.

The Conservatives' problems are more obvious. For a start, 13 years in opposition has not starved the party's backbenchers of the sort of power hunger that turned Labour into New Labour, an ideologically compromised election-winning machine. Many still believe, despite the electoral disasters of 2001 and 2005, that the route back to power lies in Thatcherism. Despite Ukip's status as a populist protest party whose low-tax policies don't correlate remotely to their ambitious spending plans, the recent upswing in the party's fortunes has rightwing Conservatives feeling worried and, more dangerously, vindicated.

Tory modernisation alienating supporters

Meanwhile, Cameron has failed to unite the party behind him. 'Modernisation', although partially successful, has alienated too much of the party by tilting towards unwinnable prizes like the Guardian newspaper, rather than the aspirational working-class vote that Thatcher won over. Different ideological factions spar over the post-Cameron party while he's still in office.

Given that he adopted a form of modernisation with which much of his party was deeply uncomfortable, Cameron's notoriously poor party management has been a major failure. As a result of it, the parliamentary Conservative Party has been wracked with rebellions in the 2010 parliament, and they don't even command a majority. In reality the Tory leadership is in a coalition with its own right wing as much as the Liberal Democrats, and it is scarcely a happier one. Whilst a small, coherent and disciplined party could achieve much, a small rabble can achieve very little indeed.

Finally, the "Big Society"' never came close to supplanting Thatcherism - whatever that word is understood to mean - as the instinct of the party, not least because not many people understood it. In fact, there is no clearer sign that Cameron is not the spiritual master of his party than that his leadership is conducted with reference to Thatcher.

Yet for all this, the government party isn't dead and can hold out hope of winning the next election. It owes this to the fact that the main party of the left is, in its own way, as divided as the right.

As with the Tories, Labour is grappling with the legacy of a hugely successful and dominant leader. Yet unlike the Conservatives, who clung to a misremembered and ideologically purist interpretation of Thatcher's legacy long after the electorate had tired of it, Labour has busied itself with the joyful rejection of what remained an election-winning creed.

Tough choices of middle ground

Dan Hodges dissects the fragmentation of the left very well here, but it can be summarised simply enough. Ed Miliband is caught between the expectations of vocal left-wing activists and his union paymasters and the need to develop a viable election strategy rooted in the tough choices of the middle ground.

Being in opposition is easy enough. The "Con Dem" government take a step, Labour condemn it. Miliband the Younger won the leadership by pitching to the powerful interest groups on the Labour left and opposing a Conservative government is something that he, like so much of the 80s Labour party, is very comfortable doing.

But winning the election will involve a sharp shift to the right. He'll need to come up with a deficit reduction strategy, which will have to both skip the punitive wealth taxes favoured by his base and feature some of the dreaded cuts. It will involve becoming much the sort of Labour party that Miliband's election to the leadership was an emphatic rejection of.

Yet even if Miliband overcomes his own instinct - which is not guaranteed - and seeks to pursue such an agenda, he's not the sort of dominant leader who could take the Labour machine with him. Leftwing activists behind the People's Assembly are already talking about the need for a new leftwing party.

It's as if we've been given a glimpse of what would have happened if Kinnock had limped to the point of a minority government in 1992: two parties carefully balanced, each too consumed by their own internal daemons to master the opposition. Today's left are no keener on seizing the centre, and with it the political initiative, than the right, and Miliband doesn't look like the leader to make them. A fact for which David Cameron ought to be very thankful.

Author of Dilettante, Henry Hill is an award winning right wing political blogger. A liberal Conservative and Unionist Party member, Henry can be found on Twitter @Dilettante11.