What would happen if World War Three started?

Most people don't know they are in a world war until the conflict is well underway – 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.

That's why some, including Ukraine's President Volodymyr Zelenskyy, say World War Three "may have already started". But, although Russia's invasion of Ukraine and conflicts in the Middle East have intensified fears of another world war, in reality, it will "only come into existence when people subjectively agree that it has", said Gavriel Rosenfield in The Washington Post.

When would we know that a Third World War had begun?

Ukraine's former military commander-in-chief, Valery Zaluzhny, said earlier this year that the war in Eastern Europe was enough for people to "absolutely believe that the Third World War has already begun".

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He pointed, for evidence, to the "direct involvement" of Russia's "autocratic allies", including North Korean soldiers appearing alongside Russian troops in the Kursk border region and "Chinese weapons" being used against Ukraine, said Politico.

However, 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 World War Three is on the way?

The world is perhaps in as tense a position as it has ever been, 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".

That is a risk that must be taken "seriously", he added, "especially over Ukraine and Taiwan".

There is some consensus that wider Russian military aggression is unlikely in the very near future but there are "visible signs of preparations across Europe". Even if further Russian incursion into Europe has seemed unlikely so far, Nato allies have "already started laying the groundwork for defences" in case that were to happen, said Ellie Cook in Newsweek.

How would World War Three be fought?

Whether it's Nato vs. Russia over Ukraine or the US vs. China over Taiwan, any conflict between the "great powers" are "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 World War Three is underway.

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". Vladimir Putin has already "threatened to use" these weapons in Ukraine, risking the further involvement of Nato forces. Their use would "completely alter the thinking of all sides".