Germany has agreed to send its Leopard 2 tank to Ukraine and allowed other countries using the German-built tank to re-export them, responding to months of pleading from Kyiv for extra firepower to break through Russian defensive lines.
Kyiv has been pushing for the Leopard 2 tank in particular because it has several advantages over the alternatives, such as Britain's Challenger 2 and the US M1 Abrams tanks.
WHY DOES UKRAINE WANT THE LEOPARD 2?
One advantage of the Leopard 2 tank is that, as well as being one of the best tanks in the West's arsenal, it is also one of the most widely used.
With some 20 nations operating the Leopard 2, several nations could each chip in a small portion of their tanks to support Ukraine. Operating a large number of one model would make it easier for Ukraine to train crew and manage maintenance.
More than 3,500 Leopard 2 tanks have been built since production began in 1978.
The tank, produced jointly by German firms Krauss-Maffei Wegmann and Rheinmetall, weighs more than 60 tonnes, has a 120 mm smoothbore gun and can hit targets at a distance of up to five km (3.1 miles).
Nations operating the Leopard include Canada, Denmark, Finland, the Netherlands, Norway, Austria, Poland, Spain, Sweden and Turkey.
HOW MANY LEOPARD 2 TANKS ARE AVAILABLE?
Although the Leopard 2 tank is widely used, tanks and other heavy weapons are in scarce supply in most of the West because many countries drastically reduced the size of their militaries after the collapse of the Soviet Union.
Germany has about 350 Leopard 2 tanks today, compared to some 4,000 battle main tanks at the height of the Cold War, according to German military expert Carl Schulze.
At the same time, it is all but impossible to buy a large amount of Leopard 2 tanks quickly.
Germany's defence industry is banned by law from producing them for stock-keeping. Typically, countries ordering new tanks need to be prepared to wait two to three years for delivery.
Even if production were ramped up, experts say it could take at least two years for the first new tanks to leave the factory.
WHAT ALTERNATIVE TANKS ARE THERE?
The United States operates thousands of M1 Abrams tanks built by General Dynamics and is poised to send dozens to Ukraine in a reversal of its previous policy, US officials say.
But the M1 Abrams are seen as unsuitable for Ukraine as they are driven by gas turbine engines with high fuel consumption that would create a challenge for Ukraine to keep them supplied even though they run can also run on diesel.
The Leopard 2 runs on a more economical engine that burns diesel, which is also easier to obtain than kerosene.
The British government announced in January it would send a squadron, or 14, of its Challenger 2 battle tanks. However, unlike the Leopard 2, it is not widely used, which limits how many can be made available for Ukraine.
In addition, unlike the M1 Abrams and the Leopard 2 which both have a 120 mm smoothbore gun, the Challenger 2 has a rifled gun with different ammunition requirements, restricting interoperability.
France has said it was considering sending its Leclerc tank, which has a 120 mm smoothbore gun, saying all options are on the table. But it has also said French overseas missions limited the number it had available to send and has said the tank's heavy-maintenance demands meant it was not ideal for Ukraine.