As Israeli warplanes and Hamas fighters trade missiles tonight, it’s worth remembering an old military truth: it is often easier to begin action than to end it.
And in this case, to know when you’ve won.
It has been conventional wisdom to say that neither Israel nor Hamas want an all-out confrontation.
We do not want war, Israel’s defence minister Ehud Barak said tonight, amid language that was acutely war-mongering on all sides.
The danger is that events will now have a momentum of their own.
Israeli army commanders say they have forces on the ground ready to move into Gaza if the order comes.
That is clearly a warning to Hamas to limit its response.
Israel says it has destroyed the militants’ longer range missiles, capable of hitting Tel Aviv.
But the loss of Ahmed Al-Jaabari, such a senior commander and key member of the leadership will have to be avenged by Hamas.
The militants have a reputation to preserve as Israel’s most potent enemy; a reputation already under threat from the more radical groups now operating out of Gaza.
If Israel’s intention is to topple Hamas, they need to think about who would replace them.
The last time Israel and Hamas were involved in a full-scale confrontation was 2008.
Operation Cast Lead saw Israeli tanks and troops move into Gaza. Hamas was subdued but not defeated.
Four years on, the Arab Spring makes it a more dangerous game for Israel.
In 2008, Hamas was hemmed in by Mubarak’s pro-West Egypt.
Now Egypt is run by the Muslim Brotherhood, Hamas’ spiritual forefathers.
Having negotiated at least a lull in the fighting, it has reacted with fury to Israel’s assault. It has withdrawn its ambassador while the Israeli counterpart in Cairo is also on his way home.
That’s gloomy news for those counting on a diplomatic channel to end the conflict.
Hamas says Israel has "opened the gates of hell". The coming hours will tell whether the two bitter enemies will walk through those gates once again.