Degree of Difficulty

Russia's decision to ship S-300 anti-aircraft missiles to Iran (with delivery date still unknown) makes Israel's ability to attack Iran's nuclear infrastructure more difficult. But not impossible.

Courtesy of the Greeks, Israel will not face an unknown air defense environment if Iran gets S-300 missiles:

The Israeli Air Force has used INIOXOS-2015, one the largest annual exercise of the Hellenic Air Force, to prepare for a potential situation where it will have to attack Iranian nuclear facilities by performing missions against the S-300PMU-1 surface-to-air missile system, stationed in Crete, according to Flightglobal.

Even a successful strike might not matter, if my speculation is correct.

Of course, experience against the S-300 would also be of use in a war against Hezbollah if Syria has any deployed to cover Lebanon's air space.

So Israel is getting ready for something. Just in case.