Initially, United States policy favored immigration without restriction.  Historically, whether immigration policies in the United States are restrictive or open have always depended on economic interests that protect American jobs.
In 1875, Congress passed the first restrictive statutes to limit immigration.  The Congressional intent was to protect Americans from the undesirable “criminals and prostitutes” that were perceived to be flooding into the United States at that time.

From 1875 to 1990, Congress viewed immigration as a threat to the American worker and local wage scales.  With each economic expansion, Congress allowed immigration levels to rise, and with each economic downturn, Congress reduced immigration levels.  In 1891, Congress created the Immigration and Naturalization Service whose duty it is to protect Americans and American jobs.

After World War I, Congress wanted to protect the American labor force from a post-war flood of immigrates from war torn Europe.  In 1921, Congress passed the Quota Law allocating immigration limits to individual countries.  The quota system still exists today.

The Great Depression had a limiting effect on immigration during the 1930’s.  Of course, World War II started an economic boom that required an increase in immigrants to supply a labor force while Americans were fighting overseas.  In response, Congress passed a treaty with Mexico for a “temporary worker” program and repealed the ban on Chinese immigration.  When the war ended and Americans returned home to work, Immigration quotas were cut back and many Mexican workers were deported.  (The 1952 act lead to “Operation Wetback” in 1954 that expelled almost 1 million Mexicans, many of whom were Americans of Spanish decent.)

In 1952, Congress passed the Immigration and Naturalization Act, making changes in the immigration laws with the intent of protecting Americans and American jobs.

 In 1986, the Immigration Reform and Control Act was passed to legalize the 6 million undocumented workers in the United States.  This marks the first Congressional action with regard to immigration that wasnot for the protection of Americans or American jobs.

It is an interesting historical footnote that Alan Greenspan, an economist and former head of the Federal Reserve Board, who had been advising U.S. Presidents since Richard Nixon and was a close advisor of Ronald Reagan, holds both a personal and professional opinion that all immigrations controls should be eliminated to allow free access to U.S. based jobs by foreign workers in order to reduce the American wage scale.

 Mr. Greenspan expounds on this position in his book “The Age of Turbulence.”  Mr. Greenspan stated in this YouTube clip that he desires the “augment integration of Skilled Labor” by importing foreign labor to “suppress wages.” http://www.youtube.com/watch?v=oqx88MyUSck&feature=related (at 3:45 to 4:40)  Mr. Greenspan also exclaims that that indivuduals, especdially Americnas, do not work for money, but, rather value their job title over the wages. he failes to explain why wages have always been the number 1 negoiated issue between employers and employees.   

 In short, the free flow of foreign workers across the U.S. boarder will depress wages for the U.S. middle class, according to the former head of the Federal Reserve, Alan Greenspan.

 It appears to be more than a coincidence that Mr. Greenspan was advising the Presidents and Congress that has allowed the free inflow of foreign workers to take jobs away from U.S. workers and to depress the wages of the American middle class.

 The Immigration Act of 1990 consisted of amendments to the 1986 Immigration Reform and Control Act.  President Bush stated in his speech that these amendments were intended to “dramatically increase the number of immigrants who may be admitted to the United States.”  George Bush: Statement on Signing the Immigration Act of 1990; http://www.presidency.ucsb.edu/ws/index.php?pid=19117#ixzz1h7ehSF5v 

Most importantly, employment related visas were for importing “nonimmigrant” workers who would “not adversely affect any currently employed U.S. worker.”  Today, every single foreign worker imported into the U.S. is intended to REPLACE a U.S. worker.

However, the Immigration and Nationalization Act at section 212(a)(5) continues to require the employer to swear that there are insufficient American workers to fill its needs, that the visa holder will be paid at or above the current wage scale; and that hiring the visa holder will not adversely affect the current American Worker.  8 CFR §214.2(h)(4)(iii).