A resistance movement is an organized effort by some portion of the civil population of a country to withstand the legally established government or an occupying power and to disrupt civil order and stability. It may seek to achieve its objectives through either the use of nonviolent resistance (sometimes called civil resistance), or the use of force, whether armed or unarmed. In many cases, as for example in Norway in the Second World War, a resistance movement may employ both violent and non-violent methods, usually operating under different organizations and acting in different phases or geographical areas within a country.
On the lawfulness of armed resistance movements in international law, there has been a dispute between states since at least 1899, when the first major codification of the laws of war in the form of a series of international treaties took place. In the Preamble to the 1899 Hague Convention II on Land War, the Martens Clause was introduced as a compromise wording for the dispute between the Great Powers who considered francs-tireurs to be unlawful combatants subject to execution on capture and smaller states who maintained that they should be considered lawful combatants.
More recently the 1977 Protocol Additional to the Geneva Conventions of 12 August 1949, and relating to the Protection of Victims of International Armed Conflicts, referred in Article 1. Paragraph 4 to armed conflicts "... in which peoples are fighting against colonial domination and alien occupation and against racist regimes..." This phraseology contains many ambiguities that cloud the issue of who is or is not a legitimate combatant.
Hence depending on the perspective of a state's government, a resistance movement may or may not be labelled a terrorist group based on whether the members of a resistance movement are considered lawful or unlawful combatants and whether they are recognized as having a right to resist occupation. Ultimately, the distinction is a political judgment.