In blockading Gaza Israel has availed itself of a well established tradition. This tactic has historically and recently raised ethical and legal questions. Of course, there is also the practical question of whether or not the blockade is an effective tactic.
What counts as an effective tactic depends, of course, on the goal in question. Israel’s overall goal seems to be to weaken and ultimately destroy Hamas. As such, the question is whether the blockade advances this goal or not.
On one hand, the blockade has had a significant impact on Gaza. The legitimate economy of Gaza seems to have been heavily damaged by the blockade and other factors. Conditions there are, to say the least, rather bad for the general population. As such, the blockade seems rather effective in hurting the general population of Gaza.
However, hurting the general population is not the same as weakening Hamas. From an economic standpoint, Hamas has apparently been able to tap into the underground economy (which is literally underground-goods are smuggled in via tunnels). This is hardly surprising: people in power are generally rather good at looking out for themselves even when the general population is suffering. This is a general problem with such blockades and economic sanctions: they seem to least hurt their true targets and most hurt the general population. This is also typical of war.
It could be argued that hurting the general population would eventually weaken Hamas. Perhaps they would eventually remove their support and even vote against Hamas. While this could happen, it is also possible that Israel’s blockade will serve instead to further harden the general population against Israel and solidify their support of Hamas.
There is also the practical concern about the impact of the blockade on world opinion. While Israel is routinely and widely condemned, Israel is also rather dependent on support from the United States and its other supporters. The blockade and the incidents that have arisen from it have strained Israel’s relations. It might turn out that this strain is merely superficial and the outrage being expressed is merely part of the political game.