The Chicago Manual of Style (CMOS) recommends capitalizing:
This differs from AP style, which only capitalizes the first and last words and major words. CMOS is more commonly used for books, journals, and other formal publications.
For short verbs like "was," the rule is straightforward: always capitalize them. Other word types have specific rules:
Many people confuse "was" with prepositions or conjunctions, but it's always a verb. The main exception is when "was" appears in a subtitle, where only the first and last words are capitalized.
Another common mistake is not capitalizing "was" when it's part of a contraction (e.g., "wasn't"). In this case, only capitalize the first letter of the contraction ("Wasn't").
Original title: "The cat was sleeping on the mat"
Correctly capitalized title: "The Cat Was Sleeping on the Mat"
Note how "was" is capitalized despite being only three letters long. This follows CMOS rules for short verbs.
Proper title capitalization makes your writing look more professional and polished. According to the University of Chicago Press, consistent title capitalization improves readability and helps readers quickly identify key information.
I once reviewed a manuscript where inconsistent capitalization of short verbs like "was" created confusion. Standardizing the capitalization improved the document's appearance and made it easier to read.