Ironically it is to Israel’s advantage that Iran is trying to build up its forces in Syria, because there the IDF has the advantage of size and proximity. Not that you would want to be forced into a war with Iran, but if you have to, its better to do it in a theater of war where Israel is another foreign force fighting it out on Syrian ground, rather than directly with Iran. Iran itself is a far larger country than Israel and has a much greater population (68 vs 8 million). But, if Iran chooses to fight it out in Syria, then it loses much of the advantage of its size and huge army. If it has to get its army to near the Israel border that means it is vulnerable to aerial attacks by the IAF, which obviously has excellent intelligence.
As last week’s clashes show, Israel claimed it had attacked all of Iran’s major facilities in Syria, including those that actually fired missiles at the Israel Golan (which none actually hit). Given that the Israeli attacks were all direct hits, the fact is that Israel came out of this major clash by far the winner. Iran now has to make choices. Should they try a small attack again, probably not worthwhile given the much greater Israeli counter-attack (in war the concept of proportional response is nonsense, see for example the US “shock and awe” campaign in Iraq). Should they try a larger counter-attack, very difficult if not impossible given the presumed damage done to their facilities in Syria.
Alternatively they could try a large missile attack from Iran itself. But, this has great danger for Iran, for two reasons, 1. Israel might then make a massive counter-attack against Iran that would be highly destructive to the regime, and 2. It could endanger Iran by having the US actually take military action in defense of its ally Israel. That’s one thing that the Ayatollahs don’t want, to give the US under Pres. Trump any excuse to exercise US military power. As he has intimidated Kim Jung-un in North Korea, with his show of force and his unpredictability, so the Ayatollahs don’t want to wake a sleeping giant.
Most likely they will try to use their proxies Hizbollah and Hamas to counter attack Israel. But, Hamas is already highly engaged in trying to get crowds of people to breach the Gaza-Israel border, and Hizbollah has just won an election in Lebanon and probably wouldn’t want to jeopardize their fragile political victory by immediately getting into a losing war with the IDF. So maybe, given little option, Iran will do nothing for a while. But, eventually after they have licked their wounds, they will try to find a way to get back at Israel.