Israel's survival hinges on having a credible, nuclear deterrent. It is possible to prevent a nuclear attack, but Israel must be alert for enemy first-strike aggression. In this case, Israel could use conventional weapons to preempt the enemy, and then resort to nuclear counterretaliation.
This is a serious dilemma for Israel. A strategic calculation of the benefits and costs of a strike would need to be made. Jerusalem's evaluation of the enemy's intentions, and the essential damage-limitation would all be factors in this calculation. A nuclear strike may be justified if the enemy has clearly stated its intentions. If Jerusalem's judgements are unclear, however, the first strike may not be the best decision.
However, you can still strike first preemptively despite the risks. As long as the strike is authorized by international law, a defensive first strike is legal. The strike could not be legal if the enemy launches a strike to interrupt Israel's battle order. The enemy may launch a preemptive attack or launch chemical or biological responses to stop the strike.
If Israel decides to attack Iran preemptively, the enemy's response will determine Israel's next steps. The preemption process would influence the enemy's response by influencing both strategic and jurisprudential assessments. Depending on the severity and extent of the provocations, the enemy's reaction would be judged rational or irrational. If the enemy's response is irrational, then the enemy could choose to strike first, thereby immobilizing Israel's order of battle.
Israel could be tempted to attack Iran first with conventional weapons and then use nuclear counter-retaliation. In such cases, the decision makers would consider whether Iran's arsenal of nuclear weapons could deter Israel, and whether Iran's counterretaliation would prevent Israel from attacking. If the enemy's response is only conventional retaliation to the attack, then the nuclear counterretaliation would not be considered.
Israel must ensure that its strategies and tactics are legal and that it follows authoritative expectations, before it preemptively attacks Iran. It would be wise to also consider Iran's reciprocal judgements. It could be an excuse for aggression if Israel decides to attack Iran preemptively and then retaliate with conventional weapons.
The nuclear strategy and arsenal of Israel would also have an effect on Jerusalem's strategic decision-making. Israel would be less likely than it is to threaten nuclear reprisals for minor territorial violations if it has nuclear weapons. Israel might not be able to threaten terrorist incursions with nuclear missiles if they do not have any nuclear weapons.
Jerusalem would also have the ability to use nuclear weapons. If the enemy had a nuke weapon arsenal, Jerusalem would probably be able. Israeli decision-makers will decide if Iran's atomic arsenal can deter Israel attacking. If Iran's nukes could deter Israel attacking, then preemptively attacking would be legal. If Iran's atomic arsenal did not deter Israel from attacking, then the decision would be illegal.