Allied countries asked Greece to send S-300 or Patriot air defense systems to Ukraine. However, Greece will not supply them because the country says it needs them for their own defense.

"Let me explain it a little more in detail: Greece has supported Ukraine in various ways, with defense material. But we have said from the first moment that we cannot dispose of weapons systems which are critical to our deterrent capability. We have surplus material with which we can support Ukraine, we have done so and I think it is the right choice. And, to the extent that we can find such material, we will continue to do so. However, the air defense systems, to which you referred, are critical systems for the protection of the Greek airspace and will not be granted to Ukraine," Prime Minister Mitsotakis said.

https://www.iefimerida.gr/politiki/mitsotakis-oykrania-zitise-s-300-patriot