The Cardassians are a humanoid race who were first seen in "The Wounded" although there had already been prior interaction between the Cardassians and the Federation. This interaction consisted of bitter, extended conflicts going back at least to the 2350s. Eventually in 2366 a shaky treaty was established between the Federation and Cardassia of which Ambassador Spock was a negotiator.
In the past, the Cardassians had been a peaceful, spiritual people. However, because of low planet resources, millions of people died from starvation and disease. Eventually a military government took power and through violent means restored prosperity to Cardassia. This military government stays in power to the present and still promotes the war effort.
In 2328 Cardassians took control over planet Bajor and, by using Bajoran forced labor, stripped the planet of it's resources. Constructing the space station Terok Nor, later known as Deep Space Nine, this mining facility it became of great commercial and strategic value when the Cardassians receded from Bajor in 2369.
Still resentful of the Cardassian occupation, the Bajorans continued their terrorist action through a group called the Maquis. In this picture a Maquis ship is firing upon a Cardassian warship. Tensions still remain along the border of Federation and Cardassian space and some of the inhabitants of the Federation outposts have joined the Maquis for vengeful purposes.