Von Neumann probes with higher acceleration were sent before the Ark to determine the most suitable planet and to begin the terraforming process if the living conditions failed to meet the necessary requirements.
Unfortunately, when the settlers reached the destination and started waking up from hibernation, a major shock came. There were no planets suitable for terraforming, only planets with no atmosphere, or Venus-like planets, or gas giants. One way or another, the settlers managed to find one planet which conditions were the closest to the ones on Earth. Alas, not close enough... The atmosphere was unbreathable due to the low levels of oxygen and high levels of carbon dioxide. The average surface temperature was too low and there were lots of substances harmful for terrestrial life forms — arsenic, cyanide, mercury...
Oddly enough, but there was life existing even in such conditions! Apparently, it was rather undeveloped, but life nonetheless. In addition, the planet was rich in minerals and, most importantly, rich in Helium-3. It is a perfect energy source.
Upon arrival the settlers faced a choice: to stay and terraform the planet calling it a new home, to fill up the Ark with Helium-3 and return, or to begin a new journey towards other stars.
Long discussions led to a split. Common settlers and workers, the future backbone of the colony's population, decided to return home. The scientists, who were to deal with the scientific issues of the expedition, insisted on continuing the journey. The military, which function was to protect the settlers from crime and hostile life forms (in case any show up), wanted to stay and continue terraforming even knowing it would take plenty of time.
The supply numbers were estimated. It showed that Helium-3 stocks were enough for only one of these tasks, and two factions needed the Ark itself. Eventually, a war has broken out…