The red line is just an arbitrary horizontal line showing the width of the "Never Again" Region at that latitude. The "Never Again" Region encompasses the continental land masses of Eurasia and Africa.

An expanded version of B.L. Montgomery's Rule 1:

Do not march on Moscow
and Rule 2:

Do not go fighting with your land armies in China


Or, viewed from a ship, US force projection should not go beyond the far littorals of the Atlantic and Pacific Oceans, absent an extremely compelling reason - which is the "but..." in the equation.

An old concept (e.g., Mahan and others of the "Guardian" school of thought). See A.T. Mahan, Naval Strategy: Compared and Contrasted with the Principles and Practice of Military Operations on Land (1911) (pdf free from Google Books).

Regards

Mike