This is part of the America's Future series airing on FOX News Channel, looking at the challenges facing the country in the 21st century.

Sheriffs in Phoenix, Ariz., have launched an aggressive crackdown on illegal aliens as part of a growing movement by local officials to help prevent illegal immigration.

Maricopa County sheriffs have instituted a zero tolerance policy — knocking on doors and arresting those suspected of living in the country illegally.

Sheriff Joe Arpaio said the law is clear cut. “Our policy is if we come across any illegal ... you are arrested. You don’t get a ticket. You get to appear before a judge later on.”

In the wake of two unsuccessful attempts at passing a comprehensive immigration reform bill in Congress, state and local governments have increasingly taken it upon themselves to find solutions to the immigration problem.

Recent statistics indicate that enforcement of existing immigration laws — both at local and state levels — has been successful in stemming the tide of illegal immigrants coming into the country.

Click here to see more on recent arrests of illegal aliens.

In 2000, 1.6 million people illegally trying to enter the United States were apprehended. This year, over half a million illegal immigrants were arrested — a sign that increased enforcement of existing laws is actually stopping immigrants from getting across the border. In fact, so much progress has been made on the state and local level that Secretary of the Department of Homeland Security Michael Chertoff thinks the border could be secure by 2011.

“For 30 years there was an image that built up that the government talked a good talk about enforcing the law against illegal immigration but that we always backed down. Now we are not backing down. Now we are standing up. We’re building the fence. We’re doubling the border patrol. We are arresting people on the interior that are illegally working,” Chertoff said

Nearly 328 miles of the border fence has already been built, and officials are hoping to construct hundreds more by year's end. Federal prosecutions of immigration offenses now amount for 57 percent of total prosecutions, eclipsing cases brought against white-collar criminals and drug offenders, which account for only 20 percent combined.

Despite these improvements, anti-illegal immigration groups say there is more to do if the government truly wants to stem the tide of illegal immigration.

According to research conducted by Numbers USA, an organization that lobbies for anti-illegal immigration legislation, nearly 21 million illegal immigrants already live in the United States, and nearly 2 million children have been born to parents who do not reside here legally. Just under 600,000 of these immigrants are considered fugitives from the law, the group said.

Other immigration experts also say enforcement alone will not solve the immigration problem, and add that the federal government needs to take more action. “If I had to grade the administration right now, I would say maybe a C plus or a C minus,” said Stephen Camarata of the Washington-based Center for Immigration Studies.

Camarata said improvement might not come in the form of new legislation but at the behest of public opinion. “Congress isn’t that interested in enforcing the law. President Bush isn’t that interested. The one interest group that has really been pushing for enforcement is the public,” said Camarata.

Although Phoenix sheriffs have had some success with their aggressive immigration policies, other public officials are less comfortable with local enforcement doing a job that they think the federal government should be handling.

In a press conference on May 1, 2008, Los Angeles mayor Antonio Villaraigosa argued that making his police officers immigration enforcers does not make economic sense. “At a time when ICE has said that they do not have the resources to go after those undocumented that are committing serious felonies, but do have the resources to go after legitimate employers, what I am saying is, we need to prioritize our resources,” said Villaraigosa.

Immigrant advocates echo Villaraigosa’s call to eliminate local law enforcement from the immigration equation.

Javier Rodriguez, a Southern California-based immigration advocate who supports full amnesty for illegal immigrants who already live in the United States, calls current policy a form of domestic terrorism that negatively affects the lives of immigrants who are working for an honest wage, even if they are not here legally.

“The Bush administration has launched a low-intensity warfare campaign of terror, of raids, of deportation, that has taken its toll on the immigrant communities throughout the country,” said Rodriguez.

But officials note that illegal immigration is a crime — and there must be consequences.

"If you break the law, you don't get a free bite at the apple," said Chertoff. "We enforce the law and deport you."

Click here to read more on America's Future.

FOX News correspondent William Lajeunesse contributed to this report.