“I am grateful to the allies for fulfilling the agreements!” Zelensky wrote on Telegram.

The United States committed 31 of the advanced battle tanks in January. A senior Ukrainian military official said “less than half” of that number have arrived. U.S. military officials have said that all 31 would arrive in the coming weeks.

Here’s what to know about the U.S. battle tank:

Why does Ukraine want the U.S. M1 Abrams?

After Russia invaded Ukraine in February 2022, Kyiv pleaded with its partners to send Western heavy battle tanks that could play in its favor on the front lines.

The M1 Abrams was a key request. The U.S. main battle tank, in service since 1980, is among the most powerful in the world. In modern versions of the vehicle, layers of depleted uranium armor provide substantial protection to the occupants.

Many Western main battle tanks are heavier and more powerful than the Soviet-era tanks and the infantry fighting vehicles Ukraine has been using. The Abrams tanks are meant to engage other tanks and break through enemy lines. And their tracks are made for moving across open terrain, similar to the landscape in southern and eastern Ukraine, the center of the ongoing counteroffensive.

How Western combat vehicles bound for Ukraine could change the war

The tanks would also help stem some of the losses Ukraine has suffered during 19 months of heavy fighting. The open-source website Oryx has documented the loss of 654 Ukrainian tanks during the war, the vast majority destroyed and others damaged or captured.

Why did the United States hesitate to send the tanks?

Despite months of requests from Ukraine, the Biden administration only announced in January that it would send the tanks. U.S. officials have said the American-made tanks, which weigh about 70 tons, present logistical difficulties for the battleground in Ukraine.

Pledges of more modern equipment for the country have also raised questions about how its allies would train Ukrainian troops to use them.

U.S. officials said that the Abrams requires complicated maintenance, logistics and special training. “It’s expensive. It’s hard to train on. … It is not the easiest system to maintain,” Undersecretary of Defense Colin Kahl told reporters after a visit to Kyiv in January.

Throughout the war, U.S. officials have also weighed carefully whether sending Kyiv a particular weapons system might provoke Russia to escalate the conflict. But after discussions with German counterparts, the Biden administration decided to donate the tanks, while Germany signed off on sending its own modern battle tank, the Leopards 2. President Biden said in January that the decision did not represent “an offensive threat to Russia” but was intended to help Ukraine defend itself from “the truly brutal aggression of Russia.”

“To liberate their land, they need to be able to counter Russia’s evolving tactics and strategy on the battlefield in the very near term,” Biden said of Ukrainian forces.

The tank donation also had symbolic value: Ukrainian officials had urged Washington to approve an Abrams delivery, arguing that it would prompt Germany to send its own tanks and break a logjam over dispatching modern battle tanks from around Europe. The German-made Leopard 2s are plentiful across Europe, but deliveries require sign-off from Germany, the manufacturer, which had been reluctant to make such a move unilaterally. The U.S. decision to send Abrams tanks appeared to have convinced Berlin, which said on Jan. 25 that it would send Leopard 2s and allow other countries to deliver theirs as well.

With some advantages, the Abrams has similar specs to the Leopard 2 tanks. The German tank is somewhat smaller than the Abrams and runs on diesel fuel, while the U.S. tank’s multi-fuel engine commonly runs on jet fuel, which can be more difficult to obtain.

Why did the M1 Abrams take so long to arrive?

While Germany’s Leopard 2 tanks arrived in Ukraine in March, it took far longer to arrange the delivery of M1 Abrams tanks.

The delay is partly because the U.S. government decided to buy the tanks from manufacturers rather than transfer them from existing stocks. They are being paid for by the Ukraine Security Assistance Initiative, a fund meant for the purchase of military equipment for Ukraine that the United States does not already have on hand. Ukrainian troops must also be trained on how to operate and maintain the Abrams and incorporate the tanks into offensive operations.

Early indications suggested the tanks would not arrive until next year or possibly later. In March, the Pentagon said it would speed delivery by sending refurbished versions of the older M1A1 tank, rather than the more advanced M1A2 tank.

The slow pace of delivery for the tanks became an issue in Congress, where senators complained that the weaponry would not be in place for Ukraine’s counteroffensive. Speaking in April, Sen. Angus King (I-Maine) said that if the M1 Abrams didn’t arrive until August or September, “it may well be too late.”

The counteroffensive began in June. So far, battle tanks have not played a decisive role in the fighting, largely because of heavy mining and the use of drones by the well-fortified Russian defense. Some experts say the tanks are likely to play a more significant role if Ukraine can punch through the main lines of defense.

In his Telegram post announcing the arrival of the tanks Monday, Zelensky said Ukraine was “already looking for new contracts” and hoped to “expand the geography of supply.”

Loveday Morris, Karen DeYoung and Dan Lamothe contributed to this report.