The Hack

Two state-sponsored hackers in Russia are believed to have broken into the Democratic National Committee servers in 2015 and 2016.

Hackers are believed to have created outlets on the internet to make the Democratic documents public. Political bloggers and newspapers reported on the hacked material.

The Rift

Leaks of documents stolen from the Democratic Congressional Campaign Committee caused a backlash against Democrats in nearly a dozen House races.

Leaked emails forced the D.N.C. chairwoman to resign and fueled a rift between supporters of Senator Bernie Sanders and Hillary Clinton.

Hacked emails from the account of Mrs. Clinton’s campaign chairman revealed the campaign’s internal dynamics and dominated the news for days.

The Response

In September, President Obama warned President Vladimir V. Putin of Russia to stay out of the American elections.

On Thursday, the Obama administration imposed sanctions on two intelligence services, four top officers and three companies in Russia.

The administration also expelled 35 Russian diplomats and their families. Officials say some may have been linked to the cyberattacks.