Timeline of Trump's delays on Russia sanctions What you need to know.

 -- Nearly four weeks after the deadline, the Trump administration has finally taken steps to implement sanctions against Russia’s defense and intelligence sectors that Congress passed with veto-proof majorities in July.

President Donald Trump was essentially forced to sign the legislation over the summer after his administration lobbied for changes to the bill.

While Congress has felt compelled to punish Russia beyond the sanctions President Barack Obama imposed at the end of his term, Trump -- who has called for warmer relations with Russia as a candidate and president -- opposed sanctions that the White House believed would only hurt cooperation with President Vladimir Putin's government.

Ahead, a timeline of Trump's opposition and how Congress forced his hand.

Senate passes bill

After word began to emerge in public that the U.S. intelligence community found Russia had interfered in the 2016 presidential election, the Obama administration slapped new sanctions on Putin's government and seized control of two Russian compounds in the U.S. But many members of Congress wanted to do more to punish Russia for its unprecedented intrusion, spurred on by increasing awareness of how extensive the campaign was and by Trump's denial of it.

That culminated in a bipartisan bill in the Senate that targeted Iran with new sanctions and was amended to include Russia. Sen. Bob Corker, the Republican chair of the Senate Foreign Relations Committee who was still publicly aligned with Trump at the time, praised it as a "strong signal to President Putin."

But the Trump administration was against the bill, especially because it gave Congress additional oversight of what is executive authority. If Trump wanted to make any changes to sanctions against Russia, he needed congressional approval.

"We would ask for the flexibility to turn the heat up when we need to, but also to ensure that we have the ability to maintain a constructive dialogue," Secretary of State Rex Tillerson told the House Foreign Affairs Committee on June 14, delicately indicating his opposition to the bill.

House makes changes before compromise

That opposition ultimately was ignored.

The House took up its own version of the bill that combined a previous piece of legislation and slapped North Korea with new sanctions as well. That bill had passed the House but it stalled in the Senate.

Despite some initial opposition from the Senate, the House passed the bill, hitting Russia, Iran and North Korea, in an overwhelming majority. The two chambers then worked together to fold North Korea into the Senate version and iron out some concerns among Republicans and the oil and gas industry over a rule in the original Senate bill that would have barred American companies and individuals from working with sanctioned Russian companies and individuals on big oil and gas projects.

What became the Countering America’s Adversaries Through Sanctions Act, or CAATSA, was passed in the House on July 25 by 419-3 and two days later in the Senate by 98-2.

Trump's angry signing statement

Trump had one week to sign the law, veto it, or do nothing and let it become law on its own.

On Aug. 2, with no public ceremony, Trump signed the bill -- but released two blistering statements with his signature, calling the legislation "significantly flawed" and saying it included "a number of clearly unconstitutional provisions."

"I built a truly great company worth many billions of dollars. That is a big part of the reason I was elected. As President, I can make far better deals with foreign countries than Congress," he said, but added he was "signing this bill for the sake of national unity."

Despite being partially "unconstitutional" in the administration's view, it also said it would fully implement the law.

"We have to follow the law, and the president signing into law is something that we certainly will follow," State Department spokesperson Heather Nauert told ABC News on Aug. 3.

Oct. 1 deadline blows by

For weeks, however, the Trump administration did not fulfill its obligations.

The law gave the Trump administration staggered deadlines to begin implementation -- the first being Oct. 1 for the Russian portion. By then, the administration was supposed to have authorized particular agencies to identify Russian defense and intelligence entities under the new sanctions that the U.S. would sanction individuals and companies for doing business with.

Shortly before the deadline, that authorization was given, but for weeks the list of Russians was missing. The State Department said that it and the Treasury were working together to develop guidance for partner countries and private companies to help them avoid violating the new sanctions, and that caused the delay.

Sens. John McCain, R-Ariz., and Ben Cardin, D-Md. -- top Republican and Democratic senators on foreign policy -- wrote a letter to the administration, wondering if they were stalling: "They’ve had plenty of time to get their act together."

"We are working to try to complete that process," Nauert said Tuesday, adding that "it’s pretty complicated, that it can take some time, that they're working to complete the process and provide the public guidance to -- certainly to the relevant people just as soon as possible."

The Iran portion of the law, by contrast, has been implemented on time so far. On Oct. 13, the administration designated the Iranian Revolutionary Guard Corps for supporting terrorism "consistent with that requirement of CAATSA," according to the Treasury.

The other Iran sanctions have an Oct. 31 date for implementation and are ready to take effect, a U.S. official told ABC News.

Guidance finally issued

Finally, after public and private prodding by Republican members of Congress, including Corker and McCain, the administration sent to Congress its list and guidance Thursday night.

After Tillerson signed off on the guidance Thursday morning, Deputy Secretary of State John Sullivan called Corker at 2:30 p.m. to relay it to him, before the department informed other senior members of Congress.

The State Department plans to release that guidance publicly "shortly," according to Nauert, who said it is also informing private industry and allied countries.

The list, obtained by ABC News, includes 39 entities ranging from aircraft and helicopter manufacturers to engineering firms, a shipyard to Russia’s spy services, the FSB and the SVR. But releasing it does not mean these sanctions have been fully implemented. While there is no asset freeze for the members of the list, individuals and businesses that are engaged in "significant transactions" with them will be subject to sanctions starting Jan. 29, 2018.

ABC News's Ben Siegel contributed to this report.