He did not let it show, contenting himself only with pointing out that that he had thought that it was in Washington that resistance to the dispatch of ground troops was the strongest. There are over ten times as many British troops in Macedonia as there are Americans.
The exchange becomes all the more significant in the light of Tony Blair's dramatic promise on Saturday that Nato would not halt its action against Serbia until the refugees had been successfully returned to their homeland in Kosovo.
The wide - and almost certainly correct - inference from his remarks has been that the Prime Minister was preparing public opinion for the prospect of a much longer haul than it has so far been given to expect. But there was another, closely related, reason for his unscheduled broadcast.
The Nato allies are now bracing themselves for a second offer - whether you choose to put quotation marks around the word is a matter of judgement - from Slobodan Milosevic designed to halt the bombing.
In the next few days there will emerge a formula - probably brokered by the Vatican, possibly with some help from the Russians - under which Mr Milosevic will pronounce himself contented with the amount of ethnic cleansing he has perpetrated, and suggest that he may be prepared to return to the negotiating table on the basis of the new - and gruesomely procured - redistribution of population in Serbia's Kosovo province.
It is probable that the moderate Albanian leader Ibrahim Rugova, now effectively a Milosevic captive, will be suborned to support the proposal.
Part of what the Prime Minister was doing, therefore, was to prepare the public for the fact that such a proposal, if it is made, will be turned down flat. Believing, as he does, that the television pictures of the forced exodus of ethnic Albanians, are what has helped to shore up Western public opinion in favour of the bombing and cement the alliance, neither he nor Clinton are about to countenance a solution which does not return them home.
The Nato leaders will say that all the offer shows is that the extraordinary precision of the Cruise missiles which hit the Ministry of Interior building but missed the hospital next door - over which the Nato command sweated long and hard because of the appalling dangers of a miss - has been effective. And that the war goes on.
Quite possibly for a long time. And this is where Mr Robertson's interesting exchange with the US legislators come in. The problems of assembling a ground force sufficiently great to enter Serbian territory with even a hope of success have not suddenly disappeared. Of these problems, the strong resistance in Washington certainly remains one of the most formidable.
The others include the difficulties of reaching agreement among all the Nato allies to provide a force which could do it. When negotiations with Milosevic finally broke down, not even the 30,000 troops that would have been needed to enforce the peace envisaged at Rambouillet had been accounted for. That figure of 30,000 was much less than would be needed to go in if war was still being waged.
That, and the notorious physical difficulties of the path into Kosovo through Macedonia, all need to be taken into account before the commitment of land forces.
The destruction of another bridge at Novi Sad in northern Yugoslavia, on the direct route from the Hungarian border to Belgrade must have made some Serbian generals wonder whether the allies were contemplating invasion from Hungary, now a Nato member. But that would mean an almost unthinkably dangerous land war on so far peaceful territory. And so on. Nevertheless, my sense is that, while ministers - after taking stock of the first fortnight - will not yet commit to large scale ground forces, they will not be so emphatic in ruling out the long-term possibility either.
One view, easily the most optimistic, is that, if and as the weather clears, much more precision bombing will begin to have its effect in Belgrade as well as in Kosovo. Some of the power elite, including the possibly doubting generals -warned daily by Mr Robertson via CNN - may revolt, fearing a summons to appear before the international court in the Hague if they blindly follow Milosevic's barbaric orders. Many of those who know the Balkans well fear this may underestimate the patriotic bunker mentality in Belgrade. Moreover the regime in Montenegro, the one Western- friendly part of the Federal Republic of Yugoslavia, is gravely at risk.
The other growing view is that bombing alone, even with improved weather, will not achieve its goal. Given the length of time it would take to assemble a ground force, that would mean more domestic problems than so far envisaged, and not only the outstanding one of British military casualties. The economic costs of the war would spiral. And there remains the problem of refugees.
If the Macedonian government continues to refuse entry from Kosovo to those without hope of admission to a third country, then Britain and other European countries may have to welcome them. Ministers do not want this to happen, because refugees soon become quasi-economic migrants unwilling to return to a wrecked, if finally, peaceful former Yugoslavia. Which is where the idea, floated by Robin Cook, for UN-protected refugee zones in the neighbouring countries comes in. But if that doesn't work you can hardly bomb to save the refugees and then let them die in camps, or worse, trying to reach the border
So the choices ahead are tougher than they looked two weeks ago. Yet the resolve of the British Cabinet seems to be remarkably firm. John Prescott, the Deputy Prime Minister, and one of those who might have been thought to have doubts, has sensibly been brought into the informal de facto war cabinet that meets daily. Moreover, the new international factor is the unexpected firmness of Germany.
But ministers throughout Nato will have to do even more to prepare public opinion for the consequences of not bringing Milosevic to his knees in the first few days of bombing. It looked at one point that Easter would be the moment that would determine whether the war would be lost or long. In London the view is that it has not been lost, but that it may be long.Reuse content