It's not, apartheid states as well as colonies are direly lacking in legitimacy.
Until 1955 Nasser was working with the israelis to achieve border security and find a non-violent solution to the rather large amount of palestinians with rather well-founded grievances that were taking advantage of the porous border. The Lavon affair occured in 1954. In February 1955 an israeli military group under Ariel Sharon attacked the egyptian army, after which Nasser changed his mind about the israelis and started actively supporting the palestinian 'fedayeen'.
Such attacks by Israel in Egypt were aimed at keeping british colonial influence alive, i.e. attempts to use the illegitimate force of another state to subjugate a neighbour. In 1973 Israel had consistently declined peace offers and made even the usians furious with their disinterest in a negotiated solution with Egypt and Syria. One should note that the cease-fire lines weren't at the israeli borders.
Around 1968 the aim of the PLO was to liberate Palestine from the jewish supremacist apartheid colony and establish a democratic state in its place, which, of course, was unacceptable to the zionist movement. The reason they resided in Lebanon was the illegal displacement of hundreds of thousands of palestinians into lebanese territory, which wasn't exactly a stabilising factor in Lebanon. Israel consistently demanded that the lebanese state should solve the problem that the israelis had forced upon it and performed 'retaliations' against lebanese civilian targets for PLO actions.
Israel also undermined lebanese political institutions by supporting christian militants, basically fomenting a civil war and trying to coerce its neighbour to install a particular government.
Consistently Israel invades, occupies, displaces and then starts to destroy homes, historical and religious sites, as well as agrarian and other civilian infrastructure. This is the main reason for Israel's dependence on conflict mines in Africa, and USA delivering oil and weaponry. More recently, as another line of revenue, Israel has developed a systematic use of its tyranny for the design and dissemination of technological tools that are used for oppressive purposes all over the world.
It's a criminal, illegitimate state predicated on the extermination of indigenous societies in its region, which is currently enacting a genocide and implicating a bunch of occidental states in it. In effect this is destroying the post-WWII international judicial order and the UN system, and will likely bring back the limitless forms of colonial and imperial violence that preceded the world wars. Russia has already taken note and ramped up its crimes against Ukraine.