President Trump departs after signing a presidential memorandum to declare the opioid crisis a national public health emergency in the East Room of the White House in Washington on Thursday, Oct. 26, 2017. (Jabin Botsford/The Washington Post)

In this occasional series, we will bring you up to speed on the biggest national security stories of the week.

The Trump dossier is one of the strangest documents to surface during a presidential election. At 35 pages, it was compiled by former British intelligence officer Christopher Steele and contains allegations suggesting that the Trump campaign had close coordination with the Kremlin. The document also includes salacious claims about President Trump himself.

Many of the allegations were already known to reporters before BuzzFeed published the dossier in January, saying “Americans can make up their own minds” about Trump, then the president-elect. Officials said some of the information in the dossier has been corroborated, but other parts — including the most salacious claims about Trump’s behavior — remain unverified.

On Tuesday, The Washington Post reported that the Hillary Clinton campaign and the Democratic National Committee helped fund the research that led to the dossier, intensifying the political fights that surround the ongoing special counsel probe into possible connections between Russia and Trump associates.

Here is a breakdown of the basics:

1. What has been reported?

In 2016, Mother Jones first reported that Democratic sources funded research efforts for the dossier. But on Tuesday, The Post went a step further, reporting that it was the Clinton campaign and the Democratic National Committee. A lawyer working on behalf of the campaign and the committee retained a Washington firm named Fusion GPS to do the research, which originally began during the Republican presidential primaries. The firm’s efforts were first supported by a Republican donor who had tasked the firm to look into Trump’s background. That individual’s identity remains unknown.

This means that Steele, the dossier’s author, came into the mix after Democrats retained the firm — a key point that those on the political right were quick to highlight.

The Post’s Philip Bump noted the dossier is made up of multiple “company intelligence reports” and that each report has an assigned number. Bump goes on to analyze a few of them.

2. What was Trump’s reaction?

On Wednesday, Trump told reporters it was a “disgrace” that Democrats helped fund the dossier, adding: “It’s a very sad commentary on politics in this country.”

Trump also called himself a victim in one of his tweets on Wednesday, attributing the quote to Fox News: “Clinton campaign & DNC paid for research that led to the anti-Trump Fake News Dossier. The victim here is the President.” @FoxNews”

Trump in the past has characterized the dossier as “phony stuff” and has demanded the Justice Department release the names of who paid for it.

Meanwhile, the Democrats are keeping mum. The Post reached out to Clinton and her top aides after the story broke, but they either refused to comment or said they were unaware of the research. But Brian Fallon, the former campaign spokesman, called it “money well spent,” and one former campaign official was quoted as saying “this is what campaigns do.”

3. Who were the key players?

Marc E. Elias, a lawyer representing the Clinton campaign and the DNC. Elias’s law firm retained the research firm Fusion GPS, which in turn hired Steele.

Perkins Coie, the law firm. Fusion GPS approached the law firm in March 2016 to offer its services — continuing the research into Trump. The law firm then engaged Fusion GPS the next month. Elias helped create the arrangement.

Christopher Steele, a former British intelligence officer. Steele has ties with the FBI and intelligence community, and has worked on Russia-related topics.

4. How much money was spent?

It’s unclear, but records from the Federal Election Commission show that the Clinton campaign paid the law firm more than $5.6 million in legal fees from June 2015 to December 2016. The DNC paid the firm $3.6 million in “legal and compliance consulting’’ since November 2015. It’s likely those legal fees include whatever Fusion GPS billed for its work.

5. How does this affect the special counsel probe?

As a legal matter, it doesn’t. The FBI and special counsel Robert S. Mueller III have known for a long time the origins of the dossier and Fusion GPS’s work. But the public knowledge that the dossier was, in essence, paid for by the Clinton campaign and the DNC could color the political debate about Trump and Russia. Republicans are seizing on the matter as further evidence that the Russia probe is flawed.

Devlin Barrett contributed to this report.