In some ways rejecting and violating even agreed-to cease-fires is a Middle East tradition.
While Israel certainly has arguments it can muster of acting in self-defense and of mutual violations in each of these cases, to greater or lesser extents, the 1948, 1956, 1967, 1973 and 1982 wars all involved some degree of Israel delaying accepting or violating cease-fires.
So the fact that a country, or in the case of Hamas, a non-state actor that controls a swath of territory, rejects a cease-fire is not per se decisive on questions of morality or legality without looking at the specific context.
Looking into the specific context, there are arguments to be made that Hamas’s rejection of the Egypt-proposed cease-fire (though at press time there were unsubstantiated reports of a possible Hamas cease-fire proposal) strengthen the legitimacy of an Israeli ground invasion (though this still may not materialize).
Paradoxically, these arguments arise from perspectives that might normally be used to criticize Israeli military actions.
The starting point of the analysis is the hotly debated separation between decisions to resort to war (jus ad bellum) and decisions about the conduct of war once it has started (jus in bello).
To many the distinction between these two issues is sacrosanct and they cannot be connected.
It would be as if an impenetrable wall were erected between the disparate moral and legal questions that come to bear in deciding to attack versus the how of the attack.
With this separation, even those criticizing the initial Israeli decision to use force would sometimes have to admit that the individual Israeli attacks (the how to use the force subsequent to the initial decision) were legal as long as those attacks followed applicable principles like necessity and proportionality.
Because of that dilemma, many critics of Israel, and of the US, have tried to blur the line between the two areas of law, arguing that if they judge Israel’s initial decision to use force as illegal, that this can affect the legality of how force is used subsequently.
Besides specific critics of Israel and the US, some philosophers of just war theory have also argued that there cannot be a complete separation between the two areas of law or morality.
While the US and Israel generally do not accept this blurring of the lines, they do have circumstances in which they speak of broader senses of self-defense and of legitimacy in justifying actions that might otherwise be harder to justify under the law of armed conflict.
Examples include many US actions in the war on terror and the rationale marshaled for the possibility of attacking Syria in 2013 and various Israeli preemptive strikes (including, according to foreign reports) on Iraq, Syria, Sudan and potentially Iran.
Returning to Hamas, in a sense, since an armed conflict is already in progress, the how of using force, whether Israel uses air power versus ground forces, is unrelated to cease-fires and will be legal as long as individual uses of force are necessary and proportional. Thus, Hamas’s rejection may make no difference.
But if one goes on the assumption either that there is not a complete separation between starting- escalating wars and their conduct, or if one looks at self-defense or legitimacy principles as separate issues, Hamas’s cease-fire rejection does seem to give Israel a windfall of greater legitimacy for an invasion.
Even Israeli critics may need to admit that regardless of whom they fault with starting this Gaza war, Israel overtly stopped fighting and was ready for a full, unconditional cease-fire.
Having tried a cease-fire rejected by Hamas, this could make the next escalating steps to obtain a cease-fire, such as an invasion, more justified.
If an invasion leads to mounting Palestinian civilian casualties, this windfall could quickly erode, but Hamas’s rejection of the cease-fire definitely alters the context of any legal-legitimacy debate going forward in Israel’s favor.