There seems to be a growing consensus that Britain needs a foreign policy. Wise voices from across the political spectrum note that when Libya stirred, David Cameron did not appear to have one, and suggest that the omission explains why he and his Government were caught out. Mr Cameron has a thousand domestic policies and his Big Society as a binding theme. In relation to events in the Middle East, he had neither a policy let alone the international equivalent of a Big Society, although goodness knows what that would be.

Perhaps as a result of the criticism, Mr Cameron acquired a foreign policy last Monday. In his Commons statement on Libya he leapt from his restrained pragmatism of the week before to becoming more hawkish than any other leader in the Western world. The Prime Minister was Blair-like in his crusading determination.

In some parts of the media Mr Cameron received plaudits for his newly robust approach, which is presumably another of the reasons for the change of emphasis. When a Prime Minister is accused of not being fully in command, there is a tendency to go over the top in displaying a form of macho leadership at the next available opportunity. Suddenly Mr Cameron had a foreign policy, and one with a populist touch. Here was a leader not to be messed with, a familiar posture in British foreign policy and one that usually ends in tears, not least for the posturing leader.

Various theories circulate as to why and how Mr Cameron suddenly acquired a hawkish foreign policy. They include his inexperience in responding to international crises, casual flexibility in relation to foreign policy, the influence of his more evangelical allies, and the need to move the media agenda on from the perceived cock-ups of the week before. Another theory is that he asked what Mr Blair would do – a commonly posed question in No 10 – and adopted his Blair-like demeanour, this time in relation to foreign affairs. Perhaps a combination of all the theories contributed to his more messianic statement on Monday.

Mr Cameron does not always read Mr Blair correctly. I doubt if the former prime minister would have made such a leap. Of greater significance is that the current occupant of No 10 is less of a free agent. Mr Blair won landslides. Mr Cameron leads a coalition in a hung parliament. Several senior Liberal Democrats were uneasy with his statement on Monday. In his Today programme interview yesterday, Paddy Ashdown chose to highlight the much better and more restrained speech Mr Cameron had made in Egypt the week before.

By yesterday, at Prime Minister's Questions, Mr Cameron's tone had changed a little, stressing merely that all eventualities must be planned for. He was on stronger ground before last Monday when he had no foreign policy. Indeed his polite Euroscepticism, unostentatious Atlanticism and determined pragmatism in the face of tumultuous events formed a more level-headed repertoire compared with his revolutionary approach to domestic policy.

When prime ministers acquire what appears to be a more principled foreign policy, it leads to trouble. Mr Blair had two that were, to some extent, contradictory. His first was to proclaim that Britain was a bridgehead linking Europe and the US. The bridge collapsed when he chose to support George W Bush over Iraq, in opposition to France and Germany. Part of the justification was Blair's 1999 Chicago speech, composed almost as spontaneously as Mr Cameron's sudden interventionism.

There is a revealing document on the current Iraq Inquiry's website. It comes from the historian Lawrence Freedman, who is a member of the Iraq Inquiry and takes the form of a memo to Jonathan Powell, Mr Blair's senior adviser, who was consulting informed outsiders in preparation for a foreign policy speech. Freedman wrote the memo in April 1999. The words are virtually the same as those in the central part of Mr Blair's Chicago speech delivered shortly afterwards, and includes the famous tests for when intervention is justified – tests with a degree of subjective flexibility that could have been used to explain why Britain had not invaded Iraq as well as justifying why it did. In a letter attached to the memo, Mr Freedman makes clear he wrote in the context of Kosovo, but Mr Blair has cited the Chicago speech ever since as an enunciation of his foreign policy.

Mr Blair's self-proclaimed foreign policy was not based on exhaustive consultation, but a memo from an historian who now questions him in an official capacity for the calamity of Iraq. But the key dimension to Mr Blair's approach was his view of Britain's relationship with the US. He was part of the "hug them close" school and there was no hugger like him.

Margaret Thatcher liked her image of an iron lady, a shrieking Eurosceptic complemented by an enduring rapport with President Reagan. In reality she signed up to all the European treaties and was an occasional critic of American foreign policy. John Major was similarly flexible, stating that Britain would be at the heart of Europe, while declaring later, and for internal party reasons, an absurd "beef war" on Europe. Messrs Wilson and Callaghan were also expedient in their variable approaches to Europe and, to some extent, the US. In their very different ways they muddled along and in doing so avoided dragging Britain into too many grandiose ventures.

Mr Cameron has been given a quick lesson in going over the top in his discovery that few other countries shared his fleeting enthusiasm for a no-fly zone over Libya and even fewer his apparent willingness to arm the insurrectionaries.

A more instructive lesson comes in the form of the latest report from the Foreign Affairs Committee on Afghanistan. Here was a war that Britain signed up to with almost universal support between the political parties. Yet after 10 years, the report suggests that the military operation is not backed by "appropriate" political leadership, Britain's rationale for continued involvement is unconvincing, and Mr Cameron's announcement of withdrawal by 2015 not fully explained either.

The former foreign secretary David Miliband has indicated that he shares the committee's concerns. The committee's report should be kept in Mr Cameron's equivalent of Mrs Thatcher's handbag in which she kept a copy of the Maastricht Treaty. The words stand as a warning to the hazards of military ventures begun quickly – and without either clear objectives or a thought-through exit strategy.

Mr Cameron seems to be not especially burdened by detail. I note that during Prime Minister's Questions he rarely consults the folder next to him. On one level the effortless command at the Despatch Box confirms a valuable capacity to extemporise. Gordon Brown went to the other extreme, wading through a mountain of notes before responding clumsily to questions.

There must be a third way. Mr Cameron's erratic improvisation in the international arena does not suggest a fascination with the complexities of policy making. I hope he reads the report of the Foreign Affairs Committee and extrapolates from it a wider conclusion about the need for pragmatic humility in international affairs, the limit of a modern British foreign policy.

s.richards@independent.co.uk; twitter.com/steverichards14