On 20 March 1956, it was the members of the Group Kraft/Oberländer who joined the CDU/CSU faction. This group was formed by members who left the Free Democratic Party (FDP) and initially functioned as a separate parliamentary group. Notably, figures like Heinrich Kraft and Gustav Oberländer played significant roles in this transition, bringing with them not only their ministerial positions but also several supporters. This move had a substantial impact on the political landscape of the time, contributing to the consolidation of political forces within the newly formed Federal Republic of Germany. For more detailed historical context, one can refer to sources such as the Wikipedia entry on "Parliamentary group (Germany)" [https://en.wikipedia.org/wiki/Parliamentary_group_(Germany)] and the academic work on Adenauer's foreign policy [https://academic.oup.com/book/1723/chapter/141334815].