The Abrams has an over engineered drive train that requires a lot of maintenance and the mechanics who are familiar with turbo jet driven tanks are few on the ground in Ukraine and only know the theory. The Leopards, on the other hand, have a drive train that isn't all that dissimilar from other tanks, including the Russian junk the Ukrainians have captured, rehabbed, and are now pointing at their former owners. That's why the Leopards are so important.
Germany wanted the US to commit Abrams tanks to the fight, but they weren't the best hardware for the job so the US balked. They didn't want to take the chance of having one disabled but not destroyed and in Russian hands, a reasonable fear since service for them is not available in Ukraine. However, the US has sent the Abrams into Poland to cover their needs when they send their Leopards into Ukraine. With the Abrams, the US has sent parts and personnel, something they really can't do for Ukraine (parts OK, personnel not).
And it needs to be remembered that Germany has sent a tremendous amount of help to Ukraine: APCs, support vehicles, towed artillery, anti aircraft weapons, you name it, other than the Leopard tanks, it has gone there.
Oh, and it;s estimated that within 10 years, Poland will be the predominant military power in Europe. The balance of power in Europe might be shifting to the east, period, and it needs to. Russia won't stop being expansionist, otherwise. Little men with big ambition will always want the Soviet empire back while ignoring the very real empire they already have. A lot of people think that because of this fixation on Europe, Russia will break apart into a loose economic federation, something else that might need to happen.
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