Ukraine war: Could Russia use tactical nuclear weapons?
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Russia's President Vladimir Putin has said he's ready to use nuclear weapons to defend Russian territory, raising the fear he might use a small, or "tactical" nuclear weapon in Ukraine.
US President Joe Biden has warned him that doing so would be the most serious military escalation since World War II.
What are tactical nuclear weapons?
Tactical nuclear weapons are small nuclear warheads and delivery systems intended for use on the battlefield, or for a limited strike.
They are designed to destroy enemy targets in a specific area without causing widespread radioactive fallout.
The smallest tactical nuclear weapons can be one kiloton or less (producing the equivalent to a thousand tonnes of the explosive TNT). The largest ones can be as big as 100 kilotons.
Strategic nuclear weapons are larger (up to 1,000 kilotons) and are launched from longer range.
By comparison, the atomic bomb the US dropped on Hiroshima in 1945 was 15 kilotons.
What tactical nuclear weapons does Russia have?
According to US intelligence, Russia has about about 2,000 tactical nuclear weapons, external.
Its tactical nuclear warheads can be placed on various types of missiles which are normally used to deliver conventional explosives, such as cruise missiles and artillery shells.
Tactical nuclear weapons can also be fired from aircraft and ships - as anti-ship missiles, torpedoes and depth charges.
The US says Russia has recently been investing heavily in these weapons to improve their range and accuracy.
Have tactical nuclear weapons ever been used before?
Tactical nuclear weapons have never been used in conflict.
Nuclear powers such as the US and Russia have found it equally effective to destroy targets on the battlefield by using modern conventional munitions.
In addition, no nuclear-armed country has so far been willing to risk unleashing all-out nuclear war by employing tactical nuclear weapons.
However, Russia might be more willing to use smaller tactical weapons than larger strategic missiles.
"They might not see it as crossing this big nuclear threshold," says Dr Patricia Lewis, head of the international security programme at the Chatham House think tank.
"They could see it as part of their conventional forces."
Russian forces can fire small nuclear warheads using conventional artillery, such as the "Malka" self-propelled gun
Are Putin's nuclear threats a real cause to worry?
In February 2022, shortly before invading Ukraine, President Putin placed Russia's nuclear forces at "special combat readiness" and held high-profile nuclear drills.
More recently, he said: "If the territorial integrity of our country is threatened, we will without a doubt use all available means to protect Russia and our people. This is not a bluff."
Russia is planning to annex the regions of southern and eastern Ukraine it has occupied after holding self-styled referendums. President Putin says he is ready to defend the "territorial integrity" of the regions "by all means."
US intelligence see this as a threat to the West not to help Ukraine try and retake these territories, rather than as a sign that he is planning a nuclear war.
But others worry that Russia, if it suffers further setbacks, might be tempted to use a smaller tactical weapon in Ukraine as a "game changer", to break a stalemate or avoid defeat.
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."
How has the US responded?
US President Joe Biden has warned Russia not to use nuclear weapons in the war in Ukraine.
Speaking during an interview with CBS News, Mr Biden said such action would "change the face of war unlike anything since World War Two," adding: "It'll be consequential."
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.
However, Russia might also be deterred from using tactical nuclear weapons by another power - China.
"Russia is heavily dependent on Chinese support," says Dr Heather Williams, nuclear expert at Kings College London.
"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."
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