By Andrew Hawkins

Only a man with herculean ambition could claim that foreign policy should dictate a nation's external events, not the other way around. Which explains why Napoleon said it.

Foreign policy causes some major headaches for modern governments because there are seldom any votes in it. Or perhaps more accurately, foreign policy presents politicians with numerous bear-traps and very few opportunities to win any votes.

The transition of power from the 39th US President to the 40th demonstrated vividly how overseas events can make or break a political career. Just minutes after Ronald Reagan was sworn in as President on 20th January 1981, some 52 hostages flew to freedom following a 444-day ordeal at the hands of the Iranian Revolutionary Government. More cynical types might think the timing more than coincidental. But the episode served to underline Reagan's brilliantly framed promise of an 'era of national renewal' after more than a decade of decline. Reagan's apparent triumph came as the US reeled from a series of disasters: the ignominious end to the Vietnam War and the shame of Watergate and Nixon, and later in the decade President Carter bungled Iran very badly indeed, presiding over first a failed hostage negotiation and then a failed rescue attempt.

British politicians have had their own foreign policy disasters too. Anthony Eden, who had enjoyed huge popularity when first becoming Prime Minister, resigned less than two years into the job after being humiliated by Nasser over the Suez Crisis. As a result of the Crisis his health suffered and he has become widely regarded as the least successful prime minister in British political history.

The more recent foreign policy casualty was, it can be argued, Tony Blair. As he left office in July 2007, 69% of the public said he would be remembered most for the Iraq conflict: one presumes they don't mean positively. It remains to be seen how Blair's time in office would have ended had he not enjoyed a colossal credit balance with the electorate including the highest leader satisfaction ratings of any Prime Minister in polling history. This resilience was brought home to me at the 2004 Labour Conference when delegates openly accused Tony Blair in fringe meetings of being a war criminal; his own side castigated him but he went on to win an historic third term less than a year later. Blair was fortunate too that few people then considered the Tories electable; as one newly-elected Conservative MP privately put it at the time, 'thank goodness Tony Blair was PM and not Iain Duncan Smith' (IDS was Opposition Leader at the time).

There are some consistent challenges thrown at governments by foreign intervention, evident throughout various conflicts including those in Iraq and Afghanistan. Governments would do well to consider these before charging into battle. Unlike the seven jus ad bellum criteria of just war theory, I believe there are three key political tests which give a good indication of whether a population will tolerate overseas intervention.

First, the proximity test: what is the link between the overseas goal and the reduction in threat to the lives of ordinary voters? In the case of Afghanistan it began as a response to 9/11 – which was fine for George W Bush because Ground Zero served as a reminder of what happens if the warning signs are ignored. Over time, however, diminishing returns set in and for Tony Blair and his successors it has become increasingly difficult for the Afghan conflict to to pass this test. Iraq was even harder to justify on this basis as voters tend to be moral relativists when it comes to overseas commitments. This makes it a very difficult call for a government, especially if there is visual or online evidence of atrocities against a civilian population. If a political leader does nothing, will risk accusations of dithering in the face of suffering, while if they intervene, they risk dissent from those of their own population who wonder why they're there in the first place.

Second, the Augustinian 'probability of success' test: people want to be on the winning side irrespective of the moral justification for war. Anthony Eden fell foul of this over Suez and Jimmy Carter over the Iranian hostage. In the case of Afghanistan the jury is still out but each time David Cameron talks about prompt withdrawal it makes it harder for this test to succeed.

Third, the resourcing test: this is an entirely internal measure of whether a government equips its armed forces sufficiently to do the job. In 2009 a ComRes/Independent poll found a massive 75% of people felt British troops in Afghanistan lacked the equipment they needed to do the job safely. This was easily past the threshold indicating trouble for Gordon Brown and should not have been a surprise because voters assume that resource adequacy is an indication of the commitment governments give to the conflict.

The leader who is notably perceived to have played and won on foreign policy is Margaret Thatcher in relation to the Falklands War. This is at least correct up to a point; by 2003 the Falklands bounce had all but disappeared. According to my theory, that conflict sort-of passed the proximity test, the strong probability of success was clearly correct, and the deployment of the Falklands Task Force in what was the first British television war, combined with Thatcher's ebullient public rhetoric, left the public in no doubt of the Government's commitment to the cause.

How does the Hawkins Test judge both Afghanistan and the current intervention in the Middle East? Both conflicts present difficulties for the proximity test and the British Government has notably downplayed the likelihood of success in the Afghan conflict – which is a mistake. The war in Afghanistan also remains dogged by doubts over resourcing, and wider defence cuts raise questions about how British intervention in the Arab Spring can be sustained.

The most important short term determinant of British public sentiment is the mood music from the Government. If the Prime Minister is perceived to be less than fully committed, for instance by talking about early force withdrawal, or if the Government is seen to fail in its duty to resource the armed forces properly, then public support for a conflict will quickly ebb away. David Cameron has so far got away with his position on Afghanistan and the Middle East but, particularly in respect of the aspiration to make swingeing defence cuts, it is not clear that he can carry public opinion with him.

Andrew Hawkins is Chairman of ComRes the polling and research organisation www.comres.com

This article was first published in Total Politics magazine and appears by kind permission of it and the author