According to John Locke's social contract theory, a government that fails to protect its citizens' natural rights would be considered 'illegal.'

According to John Locke's social contract theory, a government that fails to protect its citizens' natural rights forfeits its legitimacy and can be considered 'illegal' or illegitimate, in the sense that it no longer has the moral or just authority to rule. Locke's theory posits that individuals possess natural rights to life, liberty, and property. When people form societies and governments, they essentially enter into a social contract where they agree to abide by the common laws and governmental decisions in exchange for the protection of these natural rights.

Thus, the government is established with the consent of the governed to safeguard their rights. If the government instead becomes a threat to these rights, it breaches the social contract. The people, in turn, retain the right to revolt and establish a new government that will properly fulfill the role of protector of natural rights. This is because for Locke, the legitimacy of government is contingent upon serving the interests of the people and preserving their fundamental rights.