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Soon after Russia invaded Ukraine , President Vladimir Putin said he was moving his " deterrent forces" - Meaning nuclear weapons - to " combat ready" status.

This has raised fears that Moscow could use " tactical" nuclear weapons - not an all-out Nuclear War , but still a dramatic development.

Tactical nuclear weapons are those which could be used over relatively short distances.

This distinguishes them from " strategic" nuclear weapons. In the Cold War , these were the bombs which the two superpowers, the US and Soviet Union , could launch over long distances at each other's homeland.

The term " tactical" nevertheless incorporates many types of weapon, including smaller bombs and missiles used as " battlefield" weapons.

Russia is thought to have.

These can be placed on various types of missiles which are normally used to deliver conventional explosives.

They can even be fired as artillery shells on a battlefield.

They have also been developed for aircraft and Ships - for instance torpedoes and depth charges to target submarines.

These warheads are believed to be in storage facilities, rather than deployed and ready to fire.

But one concern is that Russia could be more willing to use smaller tactical weapons than larger strategic missiles.

" They might not see it as crossing this big nuclear threshold. They could see it as part of their conventional forces, " says Dr Patricia Lewis , head of The International security programme at the Chatham House Think Tank .

Tactical nuclear weapons vary enormously in size and power.

The smallest can be one kiloton or less (equivalent to a thousand tonnes of the explosive Tnt ) - the larger ones perhaps as big as 100 kilotons.

The Effects would depend on the size of the warhead, how far above the ground it detonates and The Local environment.

But as a comparison, Japan, during World War Two, was 15 kilotons.

Russia 's largest strategic weapons are thought to be at least 800 kilotons.

President Putin has made More Than one reference to Russia 's nuclear weapons - apparently to try to create a sense of fear.

US spies see this as a signal to The West to persuade it not to intervene more in Ukraine , not as A Sign he is planning Nuclear War .

But others worry that even though the Chances Are low, it is possible Russia , in certain conditions, might be tempted to use a smaller tactical weapon in Ukraine .

" Putin is comfortable in the 'stability-instability' world, while The West is deterred by his nuclear bluster As If Nato's billion-dollar deterrent is nothing but a Paper Tiger , " a nuclear expert with the Belfer Center for Science and International Affairs, at the Harvard Kennedy School.

US intelligence say Russia has a theory called " escalate to de-escalate" if it is in a conflict with Nato.

This involves doing something dramatic - Such as using a tactical weapon on The Battlefield , or as a demonstration Somewhere - or threatening to do so.

The idea is to frighten The Other side into backing down.

The concern is that if Putin feels cornered and that his strategy in Ukraine is failing, he could use tactical nuclear weapons as a " game changer" to break a stalemate or avoid defeat.

But The Situation would likely have to get worse in Ukraine - or back in Russia - for him to Consider This .

James Acton, a nuclear expert at The Carnegie Endowment for International Pace in Washington Dc , says: " I Am legitimately worried that in that circumstance Putin might use a Nuclear Weapon , most likely on the ground in Ukraine to terrify everyone and get His Way . We Are not at that point yet. "

Dr Heather Williams , nuclear expert at Kings College London, says one problem is that it is unclear what " winning" in Ukraine would look like for Putin - and thus what might drive Russia to use a Nuclear Weapon .

Putin claims Ukraine is part of Russia , so using nuclear weapons on its territory seems bizarre.

Russia itself is close by and " The Fallout could cross boundaries" warns Patricia Lewis .

The only time nuclear weapons have been used In Conflict was by the US at The End of World War Two against Japan. Would Putin want to become The First leader to break the taboo and use them?

Some worry he has shown a willingness to do things others thought he would not do, whether invading Ukraine or.

Dr Williams says there is a further reason why Russia might not use nuclear weapons - China .

" Russia is heavily dependent on Chinese support, but China has a 'no first use' nuclear doctrine. So if Putin did use them, it would be incredibly difficult for China to stand by him. If he used them, he would probably lose China . "

No One knows quite where the use of tactical nuclear weapons would lead. It could escalate and Putin would not want Nuclear War . But miscalculation is always a risk.

" They would imagine everyone would capitulate, " says Patricia Lewis . " What would happen is that Nato would have to come in and respond. "

The US says it is monitoring The Situation closely.

It has an extensive Intelligence Gathering machine to watch Russian nuclear activity - for instance whether tactical weapons are being moved out of storage, or if there is any change in behaviour at launch sites.

So Far , they say they have not seen any significant changes.

How the US and Nato would respond to any nuclear use is hard to predict. They may not want to escalate The Situation further and risk all-out Nuclear War but they also might want to draw a line.

This might mean a tough conventional rather than nuclear response. But what would Russia then do?

" Once you have crossed the nuclear threshold, there is no obvious stopping point, " says James Acton.

" I don't think anyone can have any confidence of what that world would look like. "

Source of news: bbc.com