Why all this talk all of a sudden about tanks?
The way the war in Ukraine is going, tanks are likely to become hugely important.
Ukraine says a big Russian offensive is coming in the spring and could throw another half a million Russian soldiers into battle.
In ground wars involving big infantry offensives, the tank can make all the difference.
Didn't experts say this war had rendered the tank obsolete?
They did, but that was about the older kind of tanks that both Ukraine and Russia were using.
They had Soviet-made T tanks from the T-64 all the way up to the T-90, the number generally denoting the year the tank first came into service.
Western-supplied anti-tank weapons combined with Ukrainian spotters using drones have destroyed more than 1,500 of them on the Russian side.
Why are the Ukrainians asking the West for more tanks?
The Ukrainians have a lot of tanks, but they're also T tanks, mainly the T-64 and T-72, which are old and vulnerable to Russian attack.
To repel a springtime Russian offensive they need modern battle tanks which are far better protected, faster and more deadly.
And they also need them if they want to break through well dug-in Russian forces and reclaim land taken by the occupiers.
What are they asking for?
The best tank for Ukraine would be the German-made Leopard 2.
Tanks are rated on their ability to strike other targets and their resilience from attack.
The Leopard 2 moves fast, has state-of-the-art targeting and is well protected by armour.
It is also, crucially, operated by 14 European countries, so spare parts are widely available.
Don't they want the Challenger tanks the British are sending?
Yes, the Challenger is a highly capable tank but the UK can only spare around a dozen of them.
Ukraine wants 300 tanks. Experts say if it gets 100 that will be enough to make a difference.
Britain is mainly offering them as a catalyst to persuade the Germans and others to send their own battle tanks.
Is that working?
Not yet. Germany has been reluctant to send the tanks but also to give permission to let other countries send them instead - which they need because Berlin holds the export licence.
Germany says that would change if the Americans send their Abrams tanks too.
Are Abrams tanks better?
It's like buying a second-hand car. Spares and servicing availability are important.
The Leopard's ubiquity in Europe gives it the edge in both departments.
The Abrams M1 also loses points on fuel consumption over its German rival and with fuel scarcity an issue that matters.
Will the US send a few anyway to encourage the Germans to release the Leopards?
As things stand, no.
They seem to want Europeans to do more to support Ukraine.
They are also worried that sending the Abrams would mean Ukraine's main offensive capability is American supplied and that would up the ante in the war, playing into Russian propaganda claims this war is about fighting US aggression.
What if Ukraine does not get the tanks it wants?
Russia's wartime leader Joseph Stalin once said "quantity has a quality all of its own" and Russia is mobilising hundreds of thousands of troops, possibly preparing for a major offensive in the spring.
Ukraine's commanders say Western-supplied tanks will be crucial to repelling that threat, and many military experts agree.
But they need them soon if they are going to have enough time to train on them and use them effectively to see off the Russian menace.