Well, first off, apologies for writing an essay in this comment, but I did a deep dive checking all my facts here.

On 26 May Nasser declared, “The battle will be a general one and our basic objective will be to destroy Israel.”

So, right off the bat, simply pointing to a seemingly offensive intent does not enter a “preemptive war” into a legal grey area under international law. The UN Charter (Article 2 (4)) simply prohibits the initiation of armed conflict, absent a UN Security Council resolution authorizing an enforcement action.

Second, this is naturally a huge oversimplification to portray the origins of the conflict as “preemptive defensive attack”. As you’ll typically find with this kind of “let’s destroy Israel for no reason!” rationale being painted over Arab countries.

The actual origin of the conflict goes back to the mid 1950s, with the Suez Crisis. Nasser nationalized the Suez Canal on July 26, 1956, prompting Western powers to try to find a way to unseat Nasser. Israel invaded the Sinai on Oct 29 1956 - the UN Security Council convened the next day, US submitted a draft resolution calling for Israel to withdraw behind the 1949 armistice lines, which Britain and France vetoed and then sent in an air attack the next day. Security Council passed resolution 119, which called for an emergency special session of the UN GA, which passed resolution 997 (ES-I), calling for an immediate ceasefire, withdrawal of all forces behind the 1949 armistice lines, arms embargo, and the reopening of the Suez Canal. Soon after, they passed resolution 1001, establishing the UNEF force to perform peacekeeping throughout the Sinai, prompting the withdrawal of British and French forces by the end of the year, and the withdrawal of Israeli forces by March 1957. This was the source of the general tension prior to the 1967 war - the “tripartite conspiracy” between Britain, France and Israel, to coerce control of the Suez Canal via an invasion of the Sinai.

Going into the 1960s - the first key event is Israel invading Jordan in the Samu Incident, on Nov 13 1966 (and, as usual, they destroyed everyone’s houses in the village of Samu). Yitzhak Rabin, then the Chief of General Staff in Israel, declared “the moment is coming when we will march on Damascus to overthrow the Syrian government” in response to the Ba’ath party coming into power in Syria and sponsoring guerilla Palestinian groups attacking Israel. Israel shot down 6 MIG-21s from the Syrian Air Force on April 7th. Moshe Dayan, Israeli Defense Minister, attested to a reporter that they were purposefully instigating clashes on the Syrian border basically by having a tractor cross territorial lines until troops on the other side became aggravated. An apparently false report was delivered on May 13 to Nasser from the USSR, that Israel had been amassing its army on the Syrian border, and the next day he ordered the troop movement into the Sinai, on May 13/14. All these things indicated a sense that Israel was escalating hostilities and prompted a defensive troop movement from Egypt, and Nasser’s vice president requested the UNEF peacekeeping force evacuate in the case of hostilities breaking out. Israeli planes, soon after (May 17/18), had fired warning shots and “buzzed” a UNEF plane to attempt to force it to land inside Israel, claiming it had violated Israel’s air space (despite that it was flying within Egyptian territory, from El Arish to Gaza). At this point, on May 23, Nasser closed the Straits of Tiran on its eastern border to Israeli ships, a contentious closure that the U.S. maintained was illegal, but predicated on agreements (namely this) Egypt was not yet a party to (see also this re: the legality) - and Nasser suggested adjudicating the issue in the ICC. Egyptian radio was publicly announcing during this general period whole period that they were on “maximum alert” for an Israeli attack. The “Waiting period”, the article you linked, describes exactly that - Egypt moved its troops into a defensive position and waited for three weeks in anticipation of an Israeli attack, which materialized as Operation Focus, a surprise attack on the air forces of Egypt, Jordan and Syria, the first clear act of hostilities.

To provide some key quotes to this point:

In another 1972 interview, Mordechai Bentov, a former member of the Israeli ruling coalition during the June war, stated: “This whole story about the threat of extermination was totally contrived and then elaborated on afterwards to justify the annexation of new Arab territories.”

Despite these moves by Nasser, Israeli Foreign Minister Abba Eban wrote in his autobiography “Nasser did not want war; he wanted victory without war”. James Reston of the New York Times wrote from Cairo on June 4th that: “Cairo does not want war and it is certainly not ready for war.” In 1968 Yitzak Rabin said: “I do not think Nasser wanted war. The two divisions he sent to the Sinai in May [1967] would not have been sufficient to launch an offensive against Israel. He knew it and we knew it.” In 1982, Israeli Prime Minister Begin admitted: “In June, 1967, we again had a choice. The Egyptian army concentrations in the Sinai did not prove that Nasser was really about to attack us. We must be honest with ourselves. We decided to attack him.” Reinforcing the position that Egypt was not prepared for war with Israel, Egypt then had 50,000 of its crack troops tied down in Yemen.

Some sources: https://www.palestinechronicle.com/the-six-day-war-and-a-possible-resolution/

https://web.stanford.edu/group/tomzgroup/pmwiki/uploads/0345-1967-06-KS-a-EYJ.pdf

https://www.youtube.com/watch?v=dy56Q1a0Flc

https://www.youtube.com/watch?v=cn3RUZsaPmg&t=1s

And yet they still happen. Expecting lands and access to them to be granted to an enemy while they remain belligerent is absurd.

I say again, annexation of land is a violation of international law, either in an offensive or defensive war. It is not a “grant”, it’s that state’s land to begin with.