Back in 1994 we convinced Ukraine, then the 3rd largest military in the world, to give up their huge arsenal of nuclear weapons, intercontinental ballistic missiles, and much of them conventional armaments, in return for a promise to protect them.
If we had not made that deal with Ukraine they would be in an entirely different situation today, and our nation would have to worry about another foreign nation capable of launching nuclear missiles anywhere they wanted to.
If we don't live up to our promise from 1994, then no nation will ever agree to reduce their nuclear arsenals in the future, and every nation will seek nuclear weapons to protect themselves.
Helping Ukraine defeat Russia will save us trillions of dollars of future defense spending, and keep out kids from having to fight against Russia when they decide to invade a NATO country.