To what does US Army Lt. General H. R. McMaster refer saying that Germany's Social Democratic Party has a 'deep and unnatural affinity' for Russia?

CNN's March 3, 2022 video H.R. McMaster on how NATO is holding up against Vladimir Putin's stress test includes the following statements by United States Army lieutenant general, Senior Fellow at the Hoover Institution and Visiting Fellow at the Freeman Spogli Institute for International Studies H. R. McMaster.

The list of accolades is intended to establish that the following words carry some weight and represent a scholarly and informed view, and may have elaborated on this point elsewhere or be referencing other known scholarly works on this topic.

My transcription starting at about 05:10:

I think that what Putin wanted more than anything was disunity. And I think if there's anything that you have to give the Biden administration credit for is that they preserved unity. They preserved unity across the Nato alliance and with other European partners, especially in the very strong response that they've had in condemning these attacks. But the big shifts that have occurred that have allowed some of the more severe economic sanctions that have really had a huge impact on Germany.

I think it's really important to note that the chancellor of Germany comes from the SPD party - the Social Democratic Party, which has a long history of kind-of a deep affinity for Russia; unnatural affinity for the Russians. And he's jettisoned that, and he's not only agreed to the more severe financial sanctions, but has also agreed to ship weapons across Germany and agreed to use - and to allow German origin weapons into Ukraine.

Question: To what does H. R. McMaster refer when he says that Germany's Social Democratic Party has a 'deep and unnatural affinity' for Russia?