What Is the Main Security Threat That Difficulty Adjustment Helps to Mitigate?

The main security threat mitigated is the "51% attack," where a single entity controls the majority of the network's hash rate. Difficulty adjustment ensures that the cost of acquiring and maintaining 51% of the hash power remains prohibitively expensive, even if the total hash rate fluctuates.

By making the mining process harder as more power joins, it maintains the integrity of the blockchain and the consensus rules.

How Does the Difficulty Adjustment Algorithm Relate to the Security Expenditure?
How Does the “Difficulty Adjustment” Mechanism Protect the Blockchain from Rapid Hash Rate Fluctuations?
What Is a “51% Attack” and Why Is Hash Rate Relevant to It?
How Can the Cost of a 51% Attack Be Calculated?
What Is “Difficulty Adjustment” in a Proof-of-Work System?
How Does the Cost of an Attack Compare to the Potential Profit from a Double-Spend?
How Does the Cost of Acquiring the Necessary Hash Power Relate to the Potential Profit from a Double-Spend?
How Does a Low Hash Rate Make a Cryptocurrency More Susceptible to a 51% Attack?