The single weapon which could help Ukraine most and the effect a European no-fly zone would have
Clarke says air power is what Ukrainians most lack for operations on the ground and that if they could create some air superiority over their own territory, it would make a "big difference".
"They need more of their own aircraft and trained pilots to use them. F-16s are fine, but they need others as well."
Clarke adds that, if Europe was prepared to do it, a no-fly zone over Ukraine could make a difference "that the Russians would find very difficult".
They would then be able to shoot down incoming ballistic missiles coming in from Russia, he says.
"If the Europeans were part of the defensive war that Ukraine were fighting, Ukrainians could use all of their air power for their offensive operations. That might make a difference on the ground in a way that the Russians would find very difficult."
What single weapon could help?
Clarke says Germany's Taurus missile could make a difference for Ukraine.
"Taurus is a better range, heavier warhead, generally, and a better warhead," he says.
"Taurus is really good at blowing up things like bridges and fixed installations. It really does a lot of damage to key installations in exactly the way you would want it to.
"It's not the sort of thing you would use against civilians, of course, but actually it's very good against military and infrastructure installations."
For context: Taurus is a powerful cruise missile system that can strike targets deep inside Russian territory, making it a potential game changer for Ukrainian forces if it could use them.
The UK and France already provide Ukraine with their own long-range missile support in the form of UK Storm Shadows and French Scalps.
Friedrich Merz, the incoming German chancellor, has proposed providing Kyiv with its Taurus missiles, something his predecessor Olaf Scholz refused to do, fearing it could drag Berlin into direct war with Moscow.
Gary C: