For the sake of argument, let us make the best case assumptions for Israel. What if the government of a new Palestinian state were somehow willing to consider itself bound by a pre-state, non-treaty demilitarization agreement? Here, even in these very improbably auspicious circumstances, the new Palestinian Arab government could still have ample pretext and opportunity for undertaking lawful treaty termination.
Palestine could withdraw from the "treaty" because of what it regarded as a "material breach," a purported violation by Israel that had allegedly undermined the object or purpose of the agreement. Or it could point toward what international law calls Rebus sic stantibus; in English, the termination doctrine known as a "fundamental change of circumstances." In this case, if Palestine should declare itself vulnerable to previously unforeseen dangers, perhaps even from the interventionary or prospectively occupying forces of other Arab armies, it could lawfully end its earlier and codified commitment to remain demilitarized.
There is another factor that explains why Netanyahu's alleged hope for Palestinian demilitarization remains misconceived. After declaring independence, a new Palestinian state government could point to any pre-independence errors of fact or to duress as perfectly appropriate grounds for agreement termination. In other words, the usual grounds that may be invoked under domestic law to invalidate contracts can also be applied under international law, whether to actual treaties or to merely treaty-like agreements.
Any treaty is void if, at the time of entry, it conflicts with a "peremptory" rule of international law, a rule accepted by the community of states as one from which "no derogation is permitted." Because the right of sovereign states to maintain military forces for self-defense is always such a rule, "Palestine" could be fully within its lawful right to abrogate any pre-independence agreement that had previously compelled its demilitarization.
It follows that Netanyahu should take no comfort from any ostensibly legal promises of Palestinian demilitarization. Indeed, should the government of any future Palestinian state choose to invite foreign armies or terrorists on to its territory, possibly even after the original government had been overthrown by more militantly Jihadist/Islamic forces, it could do so without any evident practical difficulties, and without violating relevant international law.
The core danger to Israel of any presumed Palestinian demilitarization is more practical than legal. The Washington-driven Road Map, a one-sided plan of land for nothing, stems from a persistent misunderstanding of Palestinian history and goals. At a minimum, President Barack Obama should finally understand that the Palestine Liberation Organization was formed in 1964. Significantly, this was three years before there were any "occupied territories."
What, exactly, was the PLO trying to "liberate?"
Today, facing protracted uncertainty in Cairo, Israel might soon have to deal with the Morsi regime's abrogation of the 1979 peace treaty. Jerusalem, therefore, must take prompt steps to ensure that yet another determined enemy state will not be legitimized and imposed.
Sometimes, seeing requires distance. A Palestinian state - any Palestinian state - would represent a mortal danger to Israel. This danger would not be removed, or even relieved, by any Palestinian pre-independence commitments to demilitarize.