If diplomacy is the continuation of war by other means, and if the art of diplomacy is to speak softly and carry a big stick, then no stick comes much bigger, or looks more intimidating, than a 65,000-tonne aircraft carrier. Except maybe two 65,000-tonne aircraft carriers. The tricky part of the equation is that big sticks do not come cheap.

The Government has signed a contract for two 65,000-tonne supercarriers for the Royal Navy. As big sticks go, these are the second-biggest of their kind. Only America's Nimitz class aircraft carriers come bigger.

Admiral Sir Jonathon Band, the First Sea Lord and Chief of the Naval Staff, called the order “a proud moment for the Royal Navy and a proud moment for Britain”. It would be understandable if many were wondering if it was also a sensible moment, both for the Navy and for Britain.

The cost of the two carriers is £3.9 billion. The jump-jet-style Joint Strike Fighters with which the two warships will be furnished will add £12 billion to the bill. Is this the smartest use of money from an already strained defence budget? Especially when we cannot be sure that the conflicts that may beset the world when HMS Queen Elizabeth and HMS Prince of Wales start patrolling the oceans will even be the sort that will need the support of aircraft carriers?

Are what Admiral Band calls “big-ticket” items even conscionable when Britain's Armed Forces are so stretched? The Army remains about 3,500 below strength. Servicemen can find themselves in battle with inadequate equipment. They sleep too often in dilapidated barracks. And last month General Sir Richard Dannatt, the head of the British Army, wondered loudly if it was right that a soldier who risked his life in Afghanistan was paid less than a parking meter attendant.

But the duty of the Armed Forces is to hope for the best and to prepare for the worst. Aircraft carriers fell out of fashion from the 1960s as governments tilted expenditure towards health, education and social security, and as the Cold War grew warmer. Had a war not erupted over the Falkland Islands, allowing HMS Invincible and HMS Hermes to flaunt their worth, Britain might have abandoned carriers by now.

But the peace dividend since the collapse of communism has not proved entirely peaceful. British Forces have been deployed in Iraq, Afghanistan, Sierra Leone, Kosovo and Bosnia. Aircraft carriers played vital roles in both the 1991 Gulf War and in the invasion of Iraq. Along with American troops, Britain's Armed Forces underpin Nato. Britain's new carriers will project its maritime muscle, and underline that it takes seriously its role in safeguarding global security. Last year Tony Blair told defence chiefs that in a post-September 11 world it was more important than ever for Britain to project “hard power”.

No piece of military equipment better allows Britain to project power. Aircraft carriers play to one of the key traditional strengths of the Armed Forces: the Navy. The new warships will enable Britain to play a leading role, and a complement-ary role, in fulfilling its obligations to global security and its responsibilities as a permanent member of the UN Security Council. The two carriers will not answer the question of how best to tackle terrorists in the caves of Tora Bora. But they are a downpayment worth paying on the future of the Navy and Britain's place in the world.