THE IMMIGRATION debate grips the United States. What should be done about the millions of illegal Hispanics already resident in the country? President Bush is backing a Senate Bill paving the way for many to become legal citizens. He also supports a guest-worker scheme. On the other hand, 6,000 National Guardsmen are to be posted along the Mexican border to discourage new arrivals.

Critics of the restrictive part of the bargain assert that it goes against the welcoming tradition of the US. In this, at least, they are mistaken. The open-borders policy ended in the 1880s when the Chinese were excluded. Between 1924 and 1965 the United States operated a tightly controlled immigration policy. National quotas were set. Illegal entrants faced deportation.

However, there were no quotas on migrants from the Western Hemisphere. Despite this, many Mexicans chose to avoid head taxes, visa charges and law enforcement measures by slipping into the US illegally. During the First World War, they were generally welcomed and exploited as a cheap solution to rural worker shortages. But attitudes hardened during the Great Depression and more than 400,000 were forcibly repatriated.

In 1942 another war created a new farm labour shortage. Congress offered guest-worker status to unskilled Mexican applicants. Far from destroying the rationale for illegal migration across the Rio Grande, the scheme only encouraged it and alongside the legal braceros about one million entered illegally.

In 1954 the authorities decided to get tough. Operation Wetback (as illegal Mexican workers were pejoratively known) began in Texas. The Border Patrol, assisted by federal authorities and soldiers, rounded up illegal workers in a concerted campaign of mass swoops. The hapless Mexicans were put on trucks and dumped on the far side of the border. Fearing rough treatment, many packed up and left before waiting for the strong arm of the law. The authorities claimed success, boasting that more than a million had been deported.

The figure was probably an exaggeration. In any case, the success was short-lived. In 1965, inequalities between national quotas were scrapped, which ironically had the effect of setting limits on Mexican migration for the first time. A far greater tide of illegal arrivals filled the gap.

Will President Bush’s latest proposals stem the flow? He need not search into the distant past for a clue. In 1986 Ronald Reagan gave an amnesty to illegal immigrants while tightening measures to prevent a fresh influx. It was called the Immigration Reform and Control Act. The Law of Unintended Consequences would have been more accurate. In 1986 there five million illegal immigrants in the US. Today there are twelve million.

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