Soviet spacecraft Soyuz 4 and Soyuz 5 perform the first-ever docking of manned spacecraft in orbit and the first-ever transfer of crew from one space vehicle to another (which was also the only time such a transfer was accomplished with a space walk).

This was a historic event in space exploration as it is one of the capabilities necessary to have a(n International) Space Station and we’re not going to reach the age of extra-planetary supply management (Part II and Part III) unless we can build space stations and efficiently dock spacecraft with them on inter-planetary supply runs.