Nietan became a member of the Bundestag for the second time in the
2017 German federal election. He was a member of the Committee on Foreign Affairs; in this capacity, he served as his parliamentary group's
rapporteur on the
European Union's
Common Foreign and Security Policy and relations to
Turkey. He also served on the Committee on European Affairs from 1998 until 2005 and from 2009 until 2013. In addition to his committee assignments, Nietan was part of the German-Polish Parliamentary Friendship Group. From 2005 until 2009, he chaired the German Parliamentary Friendship Group with
Belgium and
Luxembourg. Within the SPD parliamentary group, Nietan belonged to the Parliamentary Left, a left-wing movement. In the negotiations to form a
coalition government under the leadership of
Chancellor Angela Merkel following the
2017 federal elections, Nietan was part of the working group on energy, climate protection and the environment, led by
Armin Laschet,
Georg Nüßlein and
Barbara Hendricks. In the negotiations to form a so-called
traffic light coalition of the SPD, the
Green Party and the
Free Democratic Party (FDP) following the
2021 federal elections, Nietan was part of his party's delegation in the working group on foreign policy, defence, development cooperation and human rights, co-chaired by
Heiko Maas,
Omid Nouripour and
Alexander Graf Lambsdorff. In April 2024, Nietan announced that he would not stand in the
2025 federal elections but instead resign from active politics by the end of the parliamentary term. == Other activities ==