An incoming prime minister could do far worse than spare the time to see Nicholas Hytner's new production of Henry IV at the National Theatre. Not only will he enjoy a flamboyant Falstaff from Michael Gambon and, in Part One, a blazing Hotspur from David Harewood. Careworn, conscience-stricken, David Bradley's usurping king also shows that no ruler can wash away the taint of a blood-stained regime change fuelled by intrigue and deceit. The monarch who, for his former friends, "broke oath on oath, committed wrong on wrong", never does manage to move on. Distrust and rebellion in family and state alike combine to grind him down and wear him out.
For Shakespeare, the pre-emptive aggression that brought Henry Bolingbroke to power becomes a curse on the community that persists through 85 years of civil strife until the coming of the Tudors. No present-day politician could watch Henry IV and leave the theatre believing that the instigator alone ever gets to draw the line under a bitterly divisive conflict. After all, Henry begins by ordering rivals and doubters to "march all one way, and be no more opposed". Some hope: the country promptly falls apart.
Shakespeare's history plays both embodied and embedded a deep-seated national hatred of unnecessary wars, fought for ambition, vanity or short-term strategic gain. They remain a peerless school for statecraft. Next week, one other text should ideally join them in the bulging in-trays at No 10 and the FO. This is War Law by Michael Byers (Atlantic, £16.99), a lucid, quietly damning account of the supposed justifications that have topped and tailed two decades of unilateral military adventures, from Grenada to Baghdad.
A Vancouver-based legal academic, Byers has picked up advance plaudits from both Noam Chomsky and Chris Patten - a token of his armour-plated equilibrium. His book well-nigh demolishes the "dangerously destabilising doctrine of pre-emptive war" as practised by Bush and Blair. It shreds one legal fig-leaf after another worn by the go-it-alone superpower that now "wields more power than any entity since the Roman Empire". Byers starts by ripping to pieces Blair's answer to a Paxman grilling on Iraq in 2003; he ends a long way from courtroom language, with an assault on the "rule-twisting megalomaniacs" in charge of US global policy.
However, War Law makes equally bleak reading for fans of touchy-feely warfare: for anyone who, on balance, endorsed the Kosovo bombings, who hopes for a unilateral rescue in Darfur or who believes that, at the bar of history, Blair might at least call in aid the British action against a near-genocidal force in Sierra Leone. If "regime change" and similar nonsenses have little grounding in international law, neither does humanitarian, protective or "pro-democratic" intervention. Only the UN can, and should, retain the power to let slip the high-minded dogs of war.
Otherwise, the simple message is - prevent conflict first, and you won't need to intervene in the aftermath. In 2003, the 15 richest countries spent a combined $723bn on the military. That year, the world as a whole spent a measly $60bn. on (usually with-strings-attached) foreign aid.
Byers points out that global strategy in the Bush White House has sometimes seemed to come straight out of the pages of Tom Clancy's potboilers. (In one, the 1996 novel Debt of Honour, terrorists crash a jumbo into the Capitol.) The incumbent of Downing Street could do better than that for warning and wisdom. A trip to the National Theatre will at least show statesmen who boast of good faith and high ideals that their precious "honour" "hath no skill in surgery". So says Falstaff on the battlefield of Shrewsbury. He might as well speak from the clinics of Baghdad.