
Israel’s government issued a formal declaration of war against Hamas on 7th October 2023. But that doesn’t give it the right to attack Syria.
You don’t need to recognise the entity you are declaring war against as a state to do so. Cameroon declared war against Ambazonia (separatist group) on 4th December 2017.
Notably for this conflict, the First Syrian Republic declared war against Israel in 1948. While the two never agreed to a peace treaty, the First Syrian Republic no longer exists, nor its successor, nor its successor’s successor, Israel’s attack against Syria is—to put it very, very generously—of extremely dubious legality under international law. I don’t claim to be an expert in international law, but after the Assad regime was overthrown, I would consider the new Syrian Arab Republic as a legally different polity than its predecessor. I don’t recall hearing any confirmations from the new Syrian government that they want to continue war against Israel, so if that is all correct then Israel just attacked a neutral country that so happened to be populated by Arabs sympathetic to the Palestinian cause, which is honestly par for the course for the Israeli military.
In the United States, the Constitution states that in order to take your land for this purpose, you must be compensated fairly. Of course, “fairly” in terms of market value did not amount to very much, but compensation was paid and even dilapidated housing in so-called “blighted” neighbourhoods were still worth something and the cost does add up when you’re knocking hundreds of houses down and having to pay thousands for each one.