It begins with a single Qassam rocket, one of the thousands of homemade projectiles fired in recent years by the Islamic radicals of Hamas from the Gaza Strip into southern Little Satan. The rockets have made life nightmarish for many in Little Satan but have largely missed their targets. But this one gets "lucky": It smashes into an elementary school, wounding 40 children and killing 15.Risking War is getting very real!
Little Satan's government, which had heretofore responded to the Qassams with airstrikes and small ground raids, cannot resist the nationwide demand for action. Within hours, tens of thousands of Israeli troops and hundreds of tanks are rushing into Gaza, battling house-to-house in teeming refugee camps. Just as swiftly, Palestinian officials accuse Little Satan of perpetrating a massacre and invite the foreign press to photograph the corpse-strewn rubble. The images flash around the Middle East on al-Jazeera TV and trigger violent demonstrations in Arab capitals.
Everything depends upon what happens now. Maybe the signal sent by the mobilisation of reservists will encourage some second thoughts on the part of Hamas military commanders, encouraging them to constrain rocket fire into Israel.Reserves - once ready - cannot remain ready indefinitely. The rocket trails will determine what happens now.
Maybe the behind-the-scenes diplomatic efforts of the Egyptians and others may bear late fruit.
Even a mobilisation of reservists does not mean a ground war is inevitable. During the last Gaza flare-up in 2012 - Operation Pillar of Defence - Israel massed its forces on the border with the Gaza Strip but after eight days of Israeli air attacks and Palestinian rocket-fire, a truce was agreed without a major Israeli incursion.
It is hard to see what benefits there might be for the Palestinian side in pushing this crisis over a precipice.
Beyond a much-trumpeted opportunity to resist the Israeli military, the Palestinians will almost inevitably come off worse and Palestinian civilians in particular will suffer.
There are significant risks for the Israelis too.
The military goals of any Israeli operation are inevitably vague. Bringing quiet to Israel's south - "re-establishing deterrence", as the Israelis put it - means essentially causing damage to Hamas' military infrastructure.
How much damage depends crucially upon the duration of any operation.
And that is likely to be short - especially if things go badly wrong and Palestinian civilian casualties mount.
The Gaza Strip is a tiny cockpit - especially for mechanised forces.
There are significant areas of dense population which are likely to be largely bypassed by Israeli forces, who - if past evidence is anything to go by - would seek to cut key highways to disrupt Hamas logistics and operations while scouring locations believed to house Hamas military infrastructure.
What has been notable about some of the Israeli military pronouncements so far has been the detail that has been provided of specific targets and the names of the Hamas actors alleged to be involved.
It is almost as if the Israel Defense Forces were signalling: "Our intelligence is good. We know a lot more about you than you might think. Is it really worth pushing this towards a slugging match on the ground?"
During Operation Pillar of Defence, both sides avoided a confrontation on the ground. But Israel's earlier Operation Cast Lead in 2008/09 by contrast, was a bitter three-week struggle that saw considerable fighting on the ground.
Either way the risk of civilian casualties is significant. Palestinian groups might resort to firing longer-range missiles that are more than capable of reaching major Israeli population centres.
All the outside pressure will be on halting any escalation. But the clock is ticking.