If there is a declared state of war between two countries, than someone else doing what Israel did to Hezbollah would be considered a valid act of war.
The problem is that Israel and Hezbollah are currently in a state of informal hostilities but are not in a state of declared war. That distinction matters at the international level.
In the case of Gaza, Israel and Hamas were in a truce, until Hamas unilaterally broke that truce on October 7, 2023. It turns out Hamas had been using the most recent truce to build up its forces and plan those terrorist attacks, which is why Israel is unwilling to enter into a new truce. (And on a historical note, Hamas has unilaterally broken or violated every truce and peace treaty it has negotiated with Israel.) Given that the Gazan civilian population was openly sheltering and supporting the Hamas forces, the international rules of war hold that Gazan civilians are legitimate collateral damage so long as there was no intent by Israel to deliberately attack civilian targets. Importantly, Israel is formally at war with Hamas, so the legal framework is different from what happened with Hezbollah.
Nations do not declare wars anymore, because wars are illegal since WW2. Quoting Wikipedia:
> Since 1945, developments in international law such as the United Nations Charter, which prohibits both the threat and the use of force in international conflicts, have made declarations of war largely obsolete in international relations
So since then, wars are now called "counter-terrorist operations", "peace enforcement operations", "special military operations" — you name it.
Nobody "declares war" anymore. See, for a defensive war (when a nation is being attacked) there is no legal need to declare it in the first place — and this is why Ukraine does not "declare war on Russia", and this is why Israel does not declare war against Hezbollah (and Hezbollah is not even a state to begin with) — it is a defensive war from the legal standpoint.
You declare an offensive war. And this is illegal. Once you do it, there will be legal repercussions, like troubles with purchasing weapons from other countries (as they often have laws prohibiting from selling weapons to countries at war). So why would anybody do it? It would be silly.
The UN is more of a forum for continued international dialogue despite international political status (and for supporting the agendas of the world super powers), and less of a world government. Its resolutions are more flexible than a Cirque du Soleil performer, and yet I think the world is a bit better with its existence.
Israel actually didn't do that - Netanyahu literally said "this is not another skirmish, this is a war" and gave explicit goals to the war (bringing back the hostages and dismantling Hamas). Later on an additional goal was added - allowing the return of Israeli internal refugees to their homes in the North.
Yeah, but that is for "internal consumption" and it doesn't affect any international relations, unlike "declaring war".
An interesting example is the U.S. which has been in a "state of war" (official term is "period of hostility") since 2001, a.k.a. "War on Terrorism" — and it still is! Which, by the way, gives a nice bonus of immediate citizenship for signing up for the military, without the need to wait for 5 years.
The problem is that Israel and Hezbollah are currently in a state of informal hostilities but are not in a state of declared war. That distinction matters at the international level.
In the case of Gaza, Israel and Hamas were in a truce, until Hamas unilaterally broke that truce on October 7, 2023. It turns out Hamas had been using the most recent truce to build up its forces and plan those terrorist attacks, which is why Israel is unwilling to enter into a new truce. (And on a historical note, Hamas has unilaterally broken or violated every truce and peace treaty it has negotiated with Israel.) Given that the Gazan civilian population was openly sheltering and supporting the Hamas forces, the international rules of war hold that Gazan civilians are legitimate collateral damage so long as there was no intent by Israel to deliberately attack civilian targets. Importantly, Israel is formally at war with Hamas, so the legal framework is different from what happened with Hezbollah.