Sir, Christopher Caldwell ("Migration debate is out of control", April 1/2) makes several characteristically shrewd observations about the impassioned and divisive immigration debate in the US.

But with Congress he misses the central reality that, whether the legislation is tough or tender, the phenomenon of illegal immigration is here to stay. New inflows of illegal immigrants cannot be eliminated; nor can the numbers already in the United States be seriously dented by policy. The two-decades-old Immigration Control and Reform Act did not solve the problem; the new act will not either.

Take new inflows. Tougher enforcement through fences and ditches and expanding border patrols have not worked to reduce the inflows in the past two decades. The tender approach of a guestworker programme such as that proposed initially by President Bush, or any of its variants in Congress, can reduce the illegals attempting to cross the border; but the annual numbers under any such programme will be capped at 400,000 at most, leaving many desperate to get into the US any way they can. As long as borders are not thrown open, therefore, there will always be an illegal influx.

Even if the illegal inflows were miraculously decimated, a substantial stock of illegals already in the country would have to be legalised to eliminate illegals in the US. This means, of course, either the tough policy of expulsion or the tender policy of an amnesty, neither of which can be effective. Expulsion, with the aid of methods such as turning into criminal felons the illegals and even those who aid and comfort them, is embodied in the draconian House bill. But it is politically unworkable: it drew huge numbers of protesters out into the streets.

Nor will an amnesty or quasi-amnesty such as that being proposed by Senator John McCain and Senator Ted Kennedy work simply because it is crippled by so many qualifiers, to please those who oppose amnesties because they "reward" illegals who broke the law, that a sizeable fraction will not come out of the shadows to take advantage of it. A 1986 amnesty left half of an estimated 6m untouched; today, it would leave at least half of the estimated stock of 12m in the shadows.

In short, far too many of these millions would still remain illegal, and they would be continually fed by new illegals whose influx cannot be eliminated. There is no alternative therefore to putting up with the illegals.

But once this fact is confronted and digested, there is only one alternative before the American people: to treat the illegals with the humanity they deserve and which marks the traditional attitude to legal immigrants in a country built uniquely on immigration. The new immigration act should imply programmes, whose cost should be shared by Mexico when its nationals are involved, for teaching the mostly illiterate illegals the English language so they can integrate better. It should extend labour protections to illegals so they are not subject to exploitation. And it should divert the anti-terrorism expenditures away from harassment of the malnourished and uneducated illegals.

There is enough here to engage the attention of the US Congress, if it only would comprehend the problem it faces and the reforms that are truly needed.