Was the Six Days War a preemptive strike or an act of aggression?

I've done a huge writeup on this (thank you to /u/Georgy_K_Zhukov for linking it), but I've also written a paper on it separately that delves less into the history and more into the just war tradition aspect: namely, whether it's a justified pre-emptive strike.

What I've found is that most scholars, and to my knowledge nations, considered it one of the few valid pre-emptive strikes they've seen committed by others. Israel did not, for example, set out to gain land or have an incorrect cause. The government had been mostly reluctant to set out to war, urged on by the IDF which saw imminent destruction if first-strike capability wasn't exercised, and by Dayan/Weizman who agreed (but Dayan was the more turning factor, and he wasn't even in the government until June 1, by which time it's still argued war might've been inevitable). The government did not expect to have to fight Jordan, and indeed was surprised when Jordan continued to show solidarity with Egypt and Syria, having been misled by Egyptian reports of success, and so the seizing of East Jerusalem and the West Bank was not expected at all. Really, the Israelis by most accounts had expected the Jordanians to fire some token rounds to show solidarity, then back off, as they'd been more reluctant than most to start conflict with Israel.

The war was a downward tumble, and it's hard to say it was only Egypt's fault, for example, that it happened. However, regardless of what started the tumble or led to it the most, Israel still had justification for launching a pre-emptive strike because it perceived a grave threat to itself that was both imminent and extensive (especially since Egypt might've been perceiving the same thing, that means war was, again, inevitable). Egypt had mobilized its forces, and the border between Israel and Syria had been highly unstable. Egypt also blockaded the Straits of Tiran, which Israel had loudly proclaimed to be a casus belli (reason for war) in the UN General Assembly in 1957. It had expelled the UNEF (UN Emergency Force) peacekeepers from the Sinai, and signed a mutual defense pact with Jordan. Indeed there was a plan to attack Israel that Nasser was pressured out of, which would've been carried out less than 2 weeks before Israel launched its pre-emptive strike, and Nasser was pressured out of it because Israel went to the United States, who went to the Soviets, who told him to back off.

Syria had been sponsoring fedayeen activities, including funding, arming, and even directing their operations across the border. In retaliation and in pursuit of its own interests, Israel was attempting to draw Syrian fire so they could have a reason to punish Syrian forces that were trying to divert the Jordan river, which Israel was also trying to do. And there was a more general "punish Syria for the fedayeen" mentality that was also at play.

Part of the reason Egypt mobilized and did all of the above was because he saw what he thought was an imminent threat against Syria. Syria and the Soviet Union had told him multiple times before in the past months that Israeli troops were mobilizing near Syria, but in this case he believed them because of (if memory serves) the absence of tanks from the Israeli independence day parade that took place near Jerusalem (the tanks were absent to acquiesce to the fears of Jordan, though it would've violated no armistice treaty). He also felt that the Soviets were right this time because he'd gotten it from multiple people within the Soviets that war was imminent. So when he called up his forces, Israel began a partial reserve call-up, and then a full reserve call up later on. Both of those could easily have led Nasser to feel his fears were justified, or even just that he had to fight or he'd lose prestige. His pride had already been wounded greatly in spats with Jordan and the rest of the Arab world, who made fun of him for hiding behind UNEF and not directly fighting Israel. Egypt also began to allow fedayeen activities to a certain degree, which destabilized the border further, because they viewed Israel as being aggressive.

Was the strike an act of aggression? By most accounts, no. There was no motivation for it, the motivation for the strike was purely fear of annihilation and losing the element of surprise, and there were ample reasons to launch the strike. Was it pre-emptive? Yes, but in that sense both sides could've launched what would've appeared a pre-emptive strike, though Egypt's cause would've been a little weaker since it would've been viewed as a more aggressive action as it started the mobilizations (even if it felt those justified on false intelligence). Was it avoidable? Yes, as all things are. Egypt sent over an official to check out the frontline and see if Israel was massing troops, but his report of "nothing there" appears to have been lost and never communicated through. Prime Minister Eshkol of Israel offered to take the Soviet official who was accusing them of the building to the frontline himself, to look, and he offered multiple times. The Soviet official refused, reportedly saying "It is not my job to confirm, only my job to tell the truths that we Soviets know", or something to that effect. Nasser was told that a partial withdrawal of UNEF was impossible, and it was all or nothing (originally he'd simply wanted to move them out of the way to still have a reason not to launch war if necessary, but to seem in control), and could've backed down, but chose not to. Israel could've avoided a reserve call-up since it was unlikely that Nasser's troops were to attack Israel: he moved them in broad daylight and with pomp and fanfare. However, the rhetoric used, the blockade, the joint defense pact, the planned attack, the full reserve callups and movements of both sides, made the war a foregone affair by most accounts, which could've been avoided by wasn't, and Israel's pre-emptive strike has been viewed as justified.

If you want specifics on what happened, and more information (ie. the water conflict, the conflict over Dimona that I didn't mention, etc.) please take a look at the long post that was linked below and attributed to me. However, it's long (7 posts), so be ready for a long read if you take that plunge!

There had been so many geopolitical incidents between Israel and it's 3 neighbors before the war, it's hard to say whether or not Israel truly felt this the enemy movement that would start the war, but they had every reason to believe war would happen. In 1960, Egypt unexpectedly amassed a huge fighting force along Israel's southern border, while it having tense border battles with Syria. The Israeli's were totally caught off guard, suffered a huge setback in their deterrence strategy which intended to use it's intelligence capabilities to keep enemy armies from forming at the border by giving Israel advanced notice, and only backed down when it appeared both sides feared escalation. The event was probably one the most threatening military challenges Israel ever faced, a head on fight between a two large armies on two fronts.

Basically, Israel concluded this can never happen again. It is too dangerous to have an enemy army capable of massing at your border. So in 1967, when Egypt began to amass to Israel southern border, the Israeli's took no chances and went through with their preemptive strike which largely crippled their biggest opponent.

There really is so much more tension that led up to the 6 day war; closing the straits of Tiran to Israeli shipping, Egypt's removal of UN peacekeepers from the Sinai, Palestinian guerrilla attacks from Syria and the West Bank with Syrian and Egyptian support, the attempted building of a dam to block Israel access to Water in Syria and Israel's subsequent destruction of that dam, further water crisis with Syria and Jordan...But the army massing at the border with the biggest fear, that if they were already able to cross into Israel, they would not be able to stop them.