Israel is under attack, and has a right to defend itself. But the argument "They fire rockets at us; so we fire back" is not enough. A morality of equivalence is not enough in determining whether a war is just.

Classically, there are six criteria for a Just War. The first three demand that there must be just cause, good intention, and legal authority. Israel can claim all those, although lawful authority is incomplete; Israel may be a democracy, but many Palestinians in the region are second-class citizens, and there is no wider authority from the United Nations.

But there are three other criteria that are more problematic. Every other way of resolving the problem must have been tried first. There must be a reasonable chance of military success. And the means used must be in proportion to the end that the war seeks to achieve. On these, Israeli conduct is questionable.

It is not necessary to kill people to defend yourself. The Iron Dome defensive rocket shield shows that. The fact that some Israelis are sitting on the heights, eating popcorn, watching the "entertainment" below, as Gaza is bombarded is also an effective demonstration of the imbalance of power. And the key area on which the Israeli case founders is proportionality.

More than 1000 Palestinians have died, compared with 56 Israelis, only three of them civilians. More Palestinian children have been killed than Hamas fighters. The paucity of the argument that ordinary people just have to move out of the line of fire (after warnings that a bombardment is imminent) has been exposed by the shelling of the UN shelter, despite UN officials' giving the Israeli military the co-ordinates of the shelter no fewer than 12 times.

Responsibility grows commensurately with power. The Israeli army is the most powerful in the region. The country is backed by the world's greatest power, the United States. Its Secretary of State may harbour private doubts, as was shown when a microphone picked up John Kerry's off-air sarcasm about Israel's self-justifications, but Washington continues to support Israel politically and financially.

There are interesting parallels on power and responsibility. Democracies have more responsibilities than dictatorships, just as adults have more responsibilities than children; tit-for-tat does not work in parenting. States have more responsibility than individuals, which is why the terrible fiasco over botched executions in the US has such resonance: the victims' families may demand "an eye for an eye", but the state has a higher moral purpose than vengeance. It is also why it will not wash when Rome says "the percentage of paedophiles in the Church is no greater than in the general population": we expect the Church to be better than the world to which it preaches.

In Gaza, it is by no means certain that the policy of bombardment has a reasonable chance of success in Israel's stated objective of ending Hamas rockets. Uprisings in the occupied West Bank suggest that things could get worse before they get than better.

All this suggests that Israel has passed a tipping-point, and stands increasingly condemned in the court of international public opinion.

Paul Vallely is Visiting Professor in Public Ethics andMedia at the University of Chester.