Consider that it is the year 1999.
Some years back, the major powers on Earth, having determined that life exists outside our solar system, have united and created a manned, permanent base on the moon, with the purpose of maintaining surveillance of deep space, and of becoming the first outpost of Earth's defence system.
None of the aspects of this introduction made it to the series. The Earth defence system is a relic of the UFO sequel. In the series as filmed, there are international organisations, but as far as we know, the countries have not united. There is no knowledge of life in space, and the Moonbase is not military or defensive.
Copyright Martin Willey