Montesquieu points to the need for separation of powers in the state as a crucial step towards ensuring the continuation of the state.
Montesquieu sees the legitimacy of each type of government being derived in different ways. For a democratic republic, virtue is the key element of the ruler and its people, while honor is of utmost importance for a monarchy. On the other hand, despotism, which Montesquieu sees as the worst type of government, is one where the ruler is above any constraints (laws, virtue, honor), and thus constant fear is necessary to ensure the continuation of such state.
Montesquieu sees the need for a balance check for the executive power over the legislative power to ensure it can act immediately to preserve the existing laws.
For Montesquieu to create the right government, you have to consider the environment, the spirit of the people, and the telos (purpose) of the state.
Montesquieu wrote The Spirit of the Laws over 20 years
The Spirit of the Laws was written over 20 years, and his status as baron allowed him to get various political perspectives from his travels all across Europe.
Living under an absolute monarch
Montesquieu lived in an absolute monarchy, so his goal in the Spirit of the Laws was to restrict the ruler's power without rendering it useless.
French collectivism vs. British Individualism
The French were methodological collectivists because they believed that society was more than the total sum of all individuals. In contrast, the British system leaned towards a more individualistic methodology.
Natural science influences
Montesquieu's work was influenced by the rise of the modern natural sciences field, which shifted studying governments from the ideal (Plato's Republic) to the practical.
Montesquieu's types of government