President Trump has done the right thing for humanity by launching cruise missiles to strike a Syrian air base in response to the appalling chemical bombing of a rebel town on Wednesday. Common humanity demanded that something be done.

Russia's vote on the UN Security Council would ensure nothing was likely to be done from that quarter.

A coalition of force could have been organised but it would have taken time. Trump decided the strong demonstration of disapproval had to be done quickly, and he was right.

It was, in military parlance, a "proportionate response". It made the required statement to Bashar al-Assad, to his Russian backer Vladimir Putin, and to trouble-makers everywhere, not least in North Korea, that this sort of conduct will carry costs.

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It requires no further demonstration of United States disapproval for the time being and does not necessarily spell a deeper US engagement in Syria.

But the chemical attack appears to have changed Trump's attitude to the Syrian conflict. His statement on the strike yesterday included an appeal to, "all civilised nations to join us in seeking an end to the slaughter and bloodshed in Syria."

If that means he is ready to lead a concerted Western intervention on the ground to remove Assad from power, then countries such as New Zealand will have to consider the prospect carefully.

It would, of course, run hard up against the interests, and possibly the troops, of Russia. But Putin's lame response to the chemical outrage has already cost Russia its credibility as an architect of a solution in Syria.

Russian military were given warning by their US counterparts of the retaliatory strike, but if the President means to engage Assad's air and ground forces more seriously, it will be hard to avoid combat with Russian forces too.

Whether or not the missile strike leads to a deeper Western engagement, it may be a turning point in Trump's presidency.

He has discovered it is easier to be isolationist in principle than it is in practice. When you command the most powerful forces in the world and you alone have the power to punish cold-blooded cruelty, it would be hard to do nothing.

Trump seemed personally moved by the images from Khan Sheikhoun but even if he had not been moved, many of his voters would have been. For them too, isolationism is appealing in the abstract but appalling if it means war criminals can act with impunity.

Trump and his voters will see this strike as a sharp contrast to the actions of the previous President when confronted with the Syrian regime's earlier use of chemical weapons, but the contrast is not as sharp as they imagine.

In fact the retaliatory operation, cruise missiles launched from US naval ships in the Eastern Mediterranean, was exactly the operation planned by President Obama before he agreed to a Russian proposal to strip the regime of its chemical arsenal.

So much for that. Obama assuredly would not have trusted Putin a second time. Almost certainly the same action would have been taken.

If Putin was testing the limits of Trump's disengagement with foreign conflicts, he has found it.

The US is no more willing to stand by when its sympathy is aroused than it ever was. Now that this is clear, the world is a little safer.