Washington (CNN) Immigration was the centerpiece of his campaign, and 100 days into his presidency, enforcing immigration laws is one area where President Donald Trump already can claim some victories.

While Trump still has yet to achieve a single major legislative accomplishment, through a series of executive actions, administrative tweaks and aggressive rhetoric, Trump has managed to make a palpable difference on US immigration policy just more than three months into his term.

The achievements haven't been without setbacks. Trump's oft-promised wall along the border with Mexico is still without funding, and the courts have blocked implementation of other measures Trump has tried to put in place.

Still, Trump moved quickly to deliver on his promise to free immigration enforcers to crack down on undocumented immigrants, with evidence already seen in statistics coming out during his nascent administration.

In both February and March, the number of apprehensions of individuals illegally crossing the southwest border dropped sharply. While fluctuations month-to-month are normal, the drop from January to February and then again from February to March defied 17 years of trends collected by Customs and Border Protection.

Going back to 2000, not a single February or March brought a decline in apprehensions, much less a roughly 40% drop as both months brought in 2017. A source familiar with the numbers said unreleased April data was shaping up to continue the trend.

While a variety of factors influence the flow of immigrants up through Mexico to try to illegally enter the US, both in terms of conditions driving them to leave and possibilities pulling them to the US, experts attribute at least some of the drop to Trump's hardline position on immigration.

Even before many of his policies are fully implemented, experts say, the message was carried south that undocumented immigrants would be faced with more difficulties entering the US, deterring many from making the attempt -- at least in the short-term.

In addition to border crossings, early data from US Immigration and Customs Enforcement (ICE) also shows an uptick in arrests of undocumented immigrants in the US -- especially ones without a criminal record.

ICE removal authorities made 21,362 arrests in the first two months of Trump's term, including 5,441 non-criminals, according to statistics provided to CNN. Though those numbers lag behind the Obama administration's peak, they are a substantial increase over the last two years of President Barack Obama's time in office, when he instituted more discretion and prioritization in who was arrested and deported.

The Trump administration arrested roughly one-third more undocumented immigrants and more than double the number of non-criminal undocumented immigrants than the same time period in 2016.

Dan Stein, president of the anti-immigration group Federation for American Immigration Reform, said his organization gives Trump an A- grade on his first 100 days, although he would have liked to see more legislation advanced through Congress.

"You have to give the administration high grades for recruiting strong talent, for coming out of the box and setting a very strong and direct and clear tone that's had a substantial impact already in setting expectations all over the world, and you can see that on the southern border crossings," Stein told CNN.

A Republican congressional aide said that it will take some time to see the effects of Trump's policies in implementation, as many of his initiatives are still being put in place. But the border crossing numbers show that even "a tonal change" can have an impact.

"We aren't necessarily doing anything different than we've done 100 days ago, other than it's very clear or it should be very clear to people that there's a new sheriff in town and he's approaching this differently than the previous administration," the aide said on condition of anonymity to preserve the legislative process. "It's a tonal change that uncuffs border patrol agents and ICE agents ability to do what the law allows them to do. And when agents can enforce the law, we're seeing the results."

Hardline agenda implemented

One of Trump's first actions to make good on his pledge to crack down on immigration was the nomination of Alabama Sen. Jeff Sessions, a longtime advocate of hardline immigration policies during his time in the Senate, to be attorney general. Trump added Homeland Security Secretary John Kelly to the administration, as well, a former Marine general who served time in charge of Southern Command, where he became intimately familiar with cartels and violence in Central America.

In Trump's first week, he signed three executive orders taking a hard line on immigration, though his controversial travel ban order has been blocked indefinitely by the courts.

But two orders still remain largely intact -- one beefing up border security and one focused on enforcement inside the US.

Some pieces remain to be implemented, including construction of the border wall and the hiring of 10,000 new immigration officers and 5,000 new border patrol agents. Even internal estimates from US Customs and Border Protection (CBP) show that hiring thousands of new agents could take a decade.

But other elements had an immediate impact -- including the reversal of Obama administration priorities for enforcement. While the Trump administration says it is most concerned about serious criminals, the priorities for arresting undocumented immigrants in the US could include virtually every one of the estimated 11 million individuals living in the US without authorization.

One of the orders gave broad discretion to individual ICE agents and offices to determine what constitutes a threat to public safety -- and advocates and immigrant communities have described near-constant fear that even undocumented immigrants who have lived in the US for years and have citizen children and family members could be arrested and deported quickly even if they do not commit a serious crime.

One campaign pledge Trump has broken, however, has been leaving intact Obama's Deferred Action for Childhood Arrivals (DACA) program. The President has recently spoken about how sympathetic he finds the recipients of the program, who are undocumented immigrants brought to the US as children who often know no other home. The Department of Homeland Security (DHS) has continued to issue permits under the program, which requires a background check.

But advocates have also been critical of the arrests and deportations of some former recipients of DACA, some of whom DHS has said were rendered ineligible from the program despite active status because of a contested infraction.

In addition to his executive orders, the Trump administration has found ways to make subtle tweaks in policy that have major implications.

One subsidiary of DHS, US Citizenship and Immigration Services, quietly released updated lesson plans for asylum officers, which made it more difficult for would-be asylum seekers to be granted permission to stay in the US. Many of the undocumented immigrants living in the US were released on probation upon entering because they were seeking asylum, but their court dates were years in the future.

In other subtle changes, DHS expanded who might be eligible for expedited removal proceedings to be deported more quickly, and also tweaked the criteria for high-skilled visas on the day the lottery opened to enter it.

Sessions also sent guidance to federal prosecutors that could mean the Department of Justice will be going after undocumented immigrants more aggressively with criminal prosecutions, potentially clearing the way for more grounds for deportation.

Advocates for immigrants have decried Trump's first 100 days as productive, but in the wrong ways to their eyes.

"The Trump administration has launched an unprecedented attack on the American idea that we should welcome immigrants and refugees, and build a stronger country in the process," said Frank Sharry, executive director of the pro-immigrant America's Voice Education Fund in a conference call. "They are using the cover story of 'bad hombres' as a smokescreen for a strategy that declares open season on each and every undocumented immigrant in America."

The wall -- in Congress

Trump's agenda hasn't been without setbacks, however, and the only wall Trump has faced so far is one in Congress.

Despite asking for billions of dollars to begin constructing the wall, Congress is set to pass a government funding bill that runs through the fall that will not allocate any money to the border wall for this fiscal year.

And though Trump regularly spoke of a wall covering the entire border with Mexico -- and repeated claims that Mexico will somehow pay for it or reimburse the US -- Kelly has said the US will be much more strategic about putting barriers including fencing in key areas instead.

"It is unlikely that we will build a wall, a physical barrier, from sea to shining sea," Kelly told senators while testifying at a hearing this month.

In fact, the early request from the Trump administration for $1 billion toward the wall covered only 62 miles, according to documents justifying the cost obtained by CNN. Of that, only 48 miles was for new barriers, with 14 miles being replacement.

Not delivering on the wall would be a key failure by the administration, Stein said, and would undercut Trump's message with his base.

"The wall, with these innovative ideas behind design, concept, it's a glorious symbol of getting his policies done, and if it doesn't get built, it's emasculating, and he knows it," Stein said. "If he can't get the wall built, it don't look good. It doesn't look good in terms of Trump's marquis asset, which is (being) the guy who gets things done."

Stein also criticized Trump for not passing any legislation that would cut back on immigration or stiffen penalties for those who break immigration laws, blaming Republican leadership for not listening to the voters who sent Trump to the White House.

"Immigration is one of the issues that speaks right to the base of people that brought Trump to Washington," Stein said. "So Trump needs to sit down with Ryan and McConnell and have a face to face and say, 'You need to help me accomplish these promises I made to the American people.'"

Judicial setbacks

Trump's agenda has been dealt major blows by the courts, as well.

The latest slap from a federal judge was on Tuesday, when a district court in San Francisco blocked the Trump administration from implementing a piece of one of his executive orders that threatened to take federal funds away from sanctuary cities.

While the judge allowed the Justice Department to retain conditions it already had in place on some federal grants, the judge ruled that the government could not take away federal funds from cities as an effort to coerce them to cooperate with federal immigration enforcement.

The ruling had echoes of similar rulings by federal judges against Trump's two attempts at a controversial travel ban.

The first case came from the same judicial region -- with the 9th Circuit Court of Appeals upholding a block on Trump's effort to bar immigrants from certain high-risk countries and limit refugees.

That ruling forced the Trump administration back to the drawing board, but multiple federal judges again placed a block on instituting the order's second incarnation.

All of the cases are still working their way through the courts, both in terms of whether the blocks will be upheld and whether the orders will be ultimately found unconstitutional. The Trump administration has vowed to see them to the Supreme Court if necessary, where they predict they will prevail.