“Lincoln's actions were caused entirely by one obscure factor.”

This means:

The person claims that one relatively little-known factor is the complete explanation for Lincoln's actions.

The word “entirely” is crucial.

It excludes other causes.

For example, suppose someone argues:

“Lincoln's entire Civil War policy was caused by his personal relationship with Person X.”

That is an extremely strong claim.

You would need extraordinary evidence because Lincoln's actions may also have involved:

• slavery;
• constitutional questions;
• military circumstances;
• political pressures;
• public opinion;
• elections;
• Union preservation;
• economic considerations.