However, military commanders are not allowed to attack even a valid military target if the expected number of civilian casualties are disproportionate to the value of the target. This is summarized by the ICRC this way:
Launching an attack which may be expected to cause incidental loss of civilian life, injury to civilians, damage to civilian objects, or a combination thereof, which would be excessive in relation to the concrete and direct military advantage anticipated, is prohibited.This brings up the question - how does one define "excessive"?
Just as with the principle of distinction, the Prosecutor of the International Criminal Tribunal for the former Yugoslavia determined who can make that decision:
The answers to these questions are not simple. It may be necessary to resolve them on a case by case basis, and the answers may differ depending on the background and values of the decision maker. It is unlikely that a human rights lawyer and an experienced combat commander would assign the same relative values to military advantage and to injury to non-combatants. … It is suggested that the determination of relative values must be that of the “reasonable military commander”. [Final Report to the Prosecutor by the Committee established to review NATO bombings in Yugoslavia para. 50-1]
It turns out that there has been a specific ruling on a specific case being a proportional action that can shed a lot of light about whether Israel's practices fit that definition.
Wikipedia summarizes the event:
The NATO bombing of the Radio Television of Serbia headquarters occurred on 23 April 1999, during the Kosovo War. It formed part of NATO's aerial campaign against the Federal Republic of Yugoslavia, and severely damaged the Belgrade headquarters of Radio Television of Serbia (RTS). Other radio and electrical installations throughout the country were also attacked. Sixteen employees of RTS died when a single NATO missile hit the building. The television station went to air 24 hours later from a secret location.
What was NATO's justification for the attack on a TV station?
NATO Headquarters justified the bombing with two arguments; firstly, that it was necessary "to disrupt and degrade the command, control and communications network" of the Yugoslav Armed Forces, and secondly, that the RTS headquarters was a dual-use object which "was making an important contribution to the propaganda war which orchestrated the campaign against the population of Kosovo". The BBC reported that the station was targeted because of its role in Belgrade's propaganda campaign; RTS had been broadcasting Serb nationalist propaganda, which demonised ethnic minorities and legitimised Serb atrocities against them.
16 were killed to bomb what was an enemy communications hub and possibly mainly a propaganda outlet. The TV station was only out of commission for a day.
Was this proportionate?
The same International Criminal Tribunal for the former Yugoslavia ruled on that question. It first questioned whether the station was a legitimate military target, and it decided that if it was used for command and control then it was, if it was only used for propaganda it wasn't. It then goes on:
77. Assuming the station was a legitimate objective, the civilian casualties were unfortunately high but do not appear to be clearly disproportionate.
Although NATO alleged that it made "every possible effort to avoid civilian casualties and collateral damage" (Amnesty International Report, ibid, June 2000, p. 42), some doubts have been expressed as to the specificity of the warning given to civilians by NATO of its intended strike, and whether the notice would have constituted "effective warning … of attacks which may affect the civililan population, unless circumstances do not permit" as required by Article 57(2) of Additional Protocol I.
...As Western journalists were reportedly warned by their employers to stay away from the television station before the attack, it would also appear that some Yugoslav officials may have expected that the building was about to be struck. Consequently, UK Prime Minister Tony Blair blamed Yugoslav officials for not evacuating the building, claiming that "[t]hey could have moved those people out of the building. They knew it was a target and they didn’t … [I]t was probably for … very clear propaganda reasons." (ibid, citing Moral combat – NATO at war, broadcast on BBC2 on 12 March 2000).
78. Assuming the RTS building to be a legitimate military target, it appeared that NATO realised that attacking the RTS building would only interrupt broadcasting for a brief period. Indeed, broadcasting allegedly recommenced within hours of the strike, thus raising the issue of the importance of the military advantage gained by the attack vis-à-vis the civilian casualties incurred. The FRY command and control network was alleged by NATO to comprise a complex web and that could thus not be disabled in one strike. As noted by General Wesley Clark, NATO "knew when we struck that there would be alternate means of getting the Serb Television. There’s no single switch to turn off everything but we thought it was a good move to strike it and the political leadership agreed with us" (ibid, citing "Moral combat, NATO at War," broadcast on BBC2 on 12 March 2000). At a press conference on 27 April 1999, another NATO spokesperson similarly described the dual-use Yugoslav command and control network as "incapable of being dealt with in "a single knock-out blow (ibid)." The proportionality or otherwise of an attack should not necessarily focus exclusively on a specific incident. ...With regard to these goals, the strategic target of these attacks was the Yugoslav command and control network. The attack on the RTS building must therefore be seen as forming part of an integrated attack against numerous objects, including transmission towers and control buildings of the Yugoslav radio relay network which were "essential to Milosevic’s ability to direct and control the repressive activities of his army and special police forces in Kosovo" (NATO press release, 1 May 1999) and which comprised "a key element in theYugoslav air-defence network" (ibid, 1 May1999). Attacks were also aimed at electricity grids that fed the command and control structures of the Yugoslav Army (ibid, 3 May 1999).
79. On the basis of the above analysis and on the information currently available to it, the committee recommends that the OTP not commence an investigation related to the bombing of the Serbian TV and Radio Station.Clearly, if it was a military target, the warnings for the workers to leave was considered enough due care to minimize civilian casualties and the attack was completely proportional.
We see that the bar for proportionality is far lower than what Israel routinely practices. A series of tunnels or weapons caches under houses are obviously valid military targets, and as long as care is taken to minimize the loss of life, they are allowed to be targeted even if the military gain for each individual attack is relatively low and temporary.
In somewhat stark terms, the lives of 15 civilians are not considered a disproportionate price to pay for taking out a communications system for a couple of hours under international law.
Tunnels that are destroyed take weeks or months to rebuild, Rockets and weapons that are destroyed are gone forever. Yet a disruption of a few hours of a single point of a redundant communications system is considered worth the lives of 16 civilians!
By this measure, assuming that Israel's targets are valid military targets (which we already proved last time,) Israel cannot possibly be accused of violating the principle of proportionality.
(There's lots in the ICTY document that also help define how far an army can go in attacking dual-use objects, such as power plants that are used by the militants. Any flat statement that an attack on a power plant is against international law is wrong.
(Similarly, it says that while broadcasting propaganda may not be enough to consider TV station a target by itself, incitement to murder or genocide would be. Israel did not attack the Al Aqsa TV station this time but based on its genocidal nature, it appears that such an attack would be legal as well.)