Easy. The "War" - which I assume you meant Operation Protective Edge - was caused by one and only one factor: Hamas, which runs Gaza, firing rockets at Israeli civilians from Gaza territory (the indirect but root cause of the conflict is Hamas's official strategy of elimination of Israel, as for example stated by Ismail Haniye, e.g. see here)
UPDATE 1: This was just spectacularly confirmed when Israel declared unilateral 24-hr ceasefire (2014/07/27)... during which 7 more rockets were fired at Israeli cities.
UPDATE 2: Since then, HAMAS fired a barrage of rockets at Israeli cities during EVERY ceasefire, both unilateral and "mutual".
Moreover, the official stated goal of the operation is to prevent rocket attacks and tunnel-based terrorist acts out of Gaza.
All that the world needs to do to stop this war is to:
Take away all of the rockets in Gaza
Devise a reliable way to prevent more rockets from being smuggled into Gaza.
While at it, destroy all existing tunnels that Hamas dug to carry out terror acts in Israel (tunnels leading from Gaza TO Israel) - such as this one. And prevent further shipments of humanitarian aid - including money, cement and other construction materials - from being used to construct terror-purposed tunnels instead of houses for people in Gaza,
... Of course, to achieve goal #1 and #3, the world would need to do 100% of what Israel's doing in Operation Protective Edge; and to achieve #2, recreate the much-criticized blockade of Gaza that Israel is enforcing right now.
Another option to achieve #2 is to punish the countries that supply the rockets or money for rockets to Hamas (Quatar, Syria, Iran).
Another alternative to #1 and #2 is to prevent said rockets from ever being shot at Israel (just like USA has rockets but doesn't have the tendency to shoot them at Mexico or Russia). Given the official ideology and stated goals of Hamas, that may be a tad bit more difficult than ensuring they have no rockets in the first place.
References: Stratfor analysis of Israeli-Palestinian situation.