Special Provisions
It is the Union's duty to protect every State against external aggression and internal disturbance, and to ensure every State's government is carried on in accordance with the Constitution.
This is a foundational duty-imposing article (rather than itself an emergency-declaring power) that underlies Article 356's President's Rule mechanism - it establishes that the Union has an affirmative responsibility to ensure constitutional governance continues in every state, which Article 356 then gives the Union concrete tools to actually act upon when that duty requires intervention.
This article is frequently invoked alongside Article 356 as the underlying justification for central intervention in a state - the Union's Article 355 "duty" to ensure constitutional governance is what Article 356's President's Rule power is meant to help fulfil.