Adam Boulton: Tories trying to open election divide over defence, but Sir Keir Starmer says he is not Jeremy 'Stop the War' Corbyn

We have been warned. Prime Minister Rishi Sunak's pre-election pitch to voters this week was to place the nation on "war footing".

On a lightning visit to Poland and Germany, countries redolent of bloody war in Europe, he announced "a completely funded plan" to raise annual UK defence spending to 2.5% of national income over the next five to six years.

Twenty-five years ago this week another UK prime minister also had war fighting on his mind.

Ukraine-Russia war latest: Kyiv moves US tanks away from frontlines after 'hunter-killer drone' attacks

Tony Blair flew to the US to deliver one of the defining speeches of his 10 years in power. His immediate task was to persuade a reluctant President Bill Clinton to commit to NATO's defence of Kosovo against Serbian aggression.

He set it in the context of a broader ideology which became known as "humanitarian" or "liberal interventionism".

"We are all internationalists now, whether we like it or not," he told the Chicago Economic Club gathered in a dingy hotel ballroom - and a global TV audience. "We cannot turn our backs on conflicts and the violation of human rights within other countries if we want still to be secure."

The contrast in tone is stark between Mr Blair's positive argument for the use of force in some circumstances and Mr Sunak's urgent plea that "we must do more to defend our country, our interests and our values".

The UK's military options have darkened and narrowed since April 1999. A point encapsulated by Defence Secretary Grant Shapps when he observed our times have moved "from post-war to pre-war".

Mr Blair was speaking during what some called the "unipolar moment" when the US was considered to be the only global superpower, 10 years before the Iron Curtain had come down, heralding the collapse of the Soviet Union.

China seemed to be anxious to join in the world order which had been established by the Western democracies since 1945. In what were essentially wars of choice, the UK had successfully projected its forces to liberate the Falkland Islands and Saddam Hussein's Iraqi forces had been ejected from Kuwait.

Blair's view not vindicated by subsequent events

Mr Blair's Chicago speech celebrated that "our armed forces have been busier than ever - delivering humanitarian aid, deterring attacks on defenceless people, backing up UN resolutions and occasionally engaging in major wars".

His view was shared by the then United Nations secretary general Kofi Annan who had suggested UN articles could be re-examined to permit more "interventions" in an interdependent world.

For many Mr Blair's world view was not vindicated by subsequent events.

There was widespread support for the invasion of Afghanistan after the 9/11 terror attacks on America in 2001, but the UN did not endorse George W Bush and Mr Blair's expansion of the war on terror to invade Iraq.

In both cases, the interventions did not achieve their long-term aims and left behind destabilised, undemocratic countries. In both Afghanistan and Iraq, British forces failed to fulfil the military objectives which had been set for them.

Scepticism about intervention

By 2010 there was no public support in the UK or US for "boots on the ground" when instability spread to Libya and Syria, although some aerial operations continued.

In 2013 scepticism about intervention was so great the House of Commons effectively vetoed a missile response to the Assad regime's use of chemical weapons in the Syrian conflict.

When Mr Blair spoke in Chicago the so-called "peace dividend" had already been claimed.

Defence spending was down from the 4% of GNP (gross national product) it had been during the Falklands war and when the Berlin Wall fell. UK defence spending however was still comfortably above the 2% target expected of NATO members.

Pessimism growing around UK's ability to defend itself

Since the credit crunch of 2007/8, our defence spending has plunged close to that NATO minimum. Mr Sunak's announcement would only increase defence spending to the level of the Blair years.

Overall since the year 2000, the number of people employed in the British military has been cut by 30%, heading from 134,000 to 72,000 next year. The army has not been this small since the Napoleonic Wars of the 1800s.

Pessimism has grown about the UK's diminishing armed forces and our ability to defend ourselves.

In a major report this year MPs on the cross-party Defence and Security Committee reported: "The government risks being unable to build true warfighting and strategic readiness because of the sheer pace of operations, which could threaten the security of the UK. All three services have growing capability shortfalls."

UK 'no longer regarded as a top-level fighting force'

There was an outcry when the last defence secretary, Ben Wallace, revealed a senior US general told him this country "is no longer regarded as a top-level fighting force".

He and his deputy, the armed forces minister James Heappey, have endorsed Mr Sunak's plan but they are both quitting politics and believe spending should go up further than 2.5%.

Top generals claim the army is becoming too small to fulfil its functions. A former head of the British Army, Sir Patrick Sanders, has called for a significant expansion of civilian "reserve" forces.

Manpower is not everything. Advanced technology cuts the numbers needed and can enhance fighting capacity.

The UK is 29th in the world for the size of its military but rated sixth for its firepower, behind only the US, Russia, China, India and South Korea. Either way personnel and weapons cost money.

Tories trying to open up election divide

The Conservatives are trying to open up an election dividing line with Labour over defence spending.

But Sir Keir Starmer has repeatedly stressed his patriotism and his commitment to the defence of the realm. He is not Jeremy "Stop the War" Corbyn.

The Labour leadership have said "we all want 2.5%" spending but they are refusing to confirm how and when it would be paid for unless and until they are in government.

Meanwhile, Mr Sunak's spending plan has not convinced independent experts.

The Institute for Fiscal Studies doubts he could find the extra money for defence while still cutting taxes and without deep cuts in other spending.

Campaigning promises are perhaps easier to make when polling suggests the Conservatives are unlikely to have to pick up the pieces after winning the election.

Middle East events justify government choices about using armed forces

Convincing the electorate of the need to spend more on defence may not be too difficult for either Mr Sunak or Sir Keir.

In Chicago, Mr Blair seemed almost to be making a hypothetical case when he said: "We have learnt twice before in this century that appeasement does not work. If we let an evil dictator range unchallenged, we will have to spill infinitely more blood and treasure to stop him later."

Awful as subsequent events have been in the Middle East, with hindsight they did justify indisputably the choices which UK governments made about using their armed forces.

Being involved no longer a matter of choice

Now war has arrived again on European soil. Russia has launched an unprovoked attack on Ukraine and is issuing vicious threats against Ukraine's allies including the UK.

As yet NATO members are holding back from joining in the fighting. Even so, military aid for Ukraine has placed significant demands on UK defence spending. Being involved is no longer a matter of choice.

Public opinion is preparing for the worst.

This year a majority of those questioned in this country, 53%, told YouGov they expect there will be another world war in the next five to 10 years.