How will we know if we're in World War Three?

Donald Trump's decision to bomb Iran has "sparked fears Tehran could retaliate" and led to "mounting concerns that instability in the region could spark World War Three", said The Mirror.

Talk of a global conflict between the great power blocs may be reaching fever pitch, but the reality is that most people won't know they are in a world war until fighting is well under way – or so history teaches us.

The Second World War was "simply 'the War' until the late 1940s" in Britain, according to History.com, although US president Franklin D. Roosevelt "publicly declared it" as such when America entered the conflict in 1941.

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Some, Ukraine's President Volodymyr Zelenskyy among them, believe that the Third World War "may have already started". Last November, Ukraine's former military commander-in-chief, Valery Zaluzhny, pointed to the "direct involvement of Russia's autocratic allies" on the ground in Ukraine, including North Korean soldiers deployed alongside Russian troops and the use of Chinese and Iranian weapons, said Politico.

Earlier this month, Russian General Apti Alaudinov took to the Telegram social media platform to urge the Kremlin to order a mass mobilisation of a million more troops and "start preparing them for the fact that the war has already begun, which we already know is happening – World War III." But, although Russia's invasion of Ukraine and the conflict in the Middle East between Israel and Iran have intensified fears of another world war, the truth is it will "only come into existence when people subjectively agree that it has", said Gavriel Rosenfeld in The Washington Post.

When would we know the Third World War had begun?

Claims we are approaching the Third World War have long been used by leaders on all sides as a tool of diplomatic brinkmanship. Speaking to the St Petersburg economic forum last week before US air strikes on Iran, Vladimir Putin said he was worried the world was heading towards another global conflict.

Citing tit-for-tat air strikes between Israel and Iran, as well as Moscow's own war in Ukraine, the Russian president said the current geopolitical situation was "disturbing".

"I am speaking without any irony, without any jokes. Of course, there is a lot of conflict potential, it is growing, and it is right under our noses."

Multiple "system-changing conflicts with multivector loads of countries involved" are a sign that the next world war has indeed already started, UK security analyst Fiona Hill told The Guardian – if only we would recognise it.

Yet many argue that "it is not World War Three yet" until the direct involvement of Nato countries, for whom the war in Ukraine currently remains a "proxy war against the alliance's Eurasian geopolitical mega-rival", said Mark Almond in The Independent.

In the end, we may not really know the Third World War has happened until it enters "the history books as a real event in retrospect", said Rosenfield.

Are there signs that the Third World War is on the way?

"The question of how a third world war might erupt haunts us today more than at any time since the end of the last world war," said author Richard Overy in The Telegraph.

The world is perhaps in as tense a position as it has been in nearly a century and countries are more readily equipped with stockpiles of highly destructive weapons than they were before the previous wars.

The "major powers" holding nuclear weapons make the "situation far more perilous", Dr David Wearing, lecturer in international relations at the University of Sussex, told Sky News. This is not because one side is more likely to make a "premeditated decision to spark the apocalypse" but that, if one side "misinterprets" the action of the other, a "nuclear exchange begins, despite the fact that no one was looking for one".

This is a view also held by citizens across Europe and America. A YouGov poll published in May found nearly half of people on both sides of the Atlantic believe another global conflict is likely to occur in the next five to ten years.

The vast majority of those polled expect any future global conflict would involve the use of nuclear weapons. This would lead to a higher casualty count than in previous world wars, with a sizeable minority thinking it would lead to the deaths of most people on the planet.

How would the Third World War be fought?

Whether it's Nato vs. Russia over Ukraine or the US vs. China over Taiwan, any conflict between the "great powers" is "rarely settled in a neat, tidy fashion", said Brian Kerg at the New Atlanticist.

Wars between world superpowers are usually "long, gruelling slogs of attrition", which then "tend to expand horizontally, ensnaring other regions in their wake". That is possibly the clearest sign that the Third World War is under way.

Nuclear arms, of course, present the risk of conflicts being over in a rapid flash of mass destruction. The "tearing up" of treaties that capped the spread of nuclear arms means more countries than ever are now in a position to build a nuclear arsenal: a sign that the "old nuclear order is dead", said Richard Spencer in The Times, and the "only thing preventing catastrophe is, once again, mutually assured destruction".

That could mean nuclear arms are more likely to be used as "tactical weapons", rather than the all-out bombings of Hiroshima and Nagasaki in 1945. Smaller nuclear weapons could be "fired from artillery or tanks, or attached to short-range missiles".

The scenario of Putin using tactical nuclear weapons in Ukraine, Iran building a nuclear bomb and using it on Israel, or China deploying them against Taiwan are all "possible" if still not "probable", said Overy.

"Predicting – more accurately, imagining – the wars of the future can produce dangerous fantasies that promote anxiety over future security. It is likely that even the most plausible prognosis will be wrong."