Single-party states are justified for a number of reasons. Most often, proponents of a single-party state argue that the existence of separate parties runs counter to national unity. Others argue that the single party is the vanguard of the nation, and therefore its right to rule cannot be legitimately questioned.
Some single party states only outlaw opposition parties, while allowing subordinate allied parties to exist as part of a permanent coalition such as a popular front. Examples of this are the People's Republic of China under the United Front, or the National Front in former East Germany. Others may allow non-party members to run for legislative seats, as was the case with Taiwan's Tangwai movement in the 1970s and 1980s.
Within their own countries, dominant parties ruling over single-party states are often referred to simply as the Party. For example, in reference to the Soviet Union, the Party meant the Communist Party of the Soviet Union; in reference to the former People's Republic of Poland it referred to the Polish United Workers' Party.
Most single-party states have been ruled either by parties following the ideology of Marxism-Leninism or international solidarity such as the Soviet Union for most of its existence, by parties following some type of nationalist or fascist ideology such as Germany under Adolf Hitler, or parties that came to power in the wake of independence from colonial rule. One-party systems often arise from decolonization because one party has had an overwhelmingly dominant role in liberation or in independence struggles. Not all authoritarian states and dictatorships operate based on single-party rule. Some, especially absolute monarchies and certain military dictatorships, have made all political parties illegal.