Does a Force to Be Reckoned with?

The verb phrase reckon with means to deal with, prepare for, or take into account. So, a force to be reckoned with is someone or something that should not be ignored or dealt with lightly. The phrase is used to refer to people or things that are powerful, influential, and maybe even dangerous.

What does the quote force to be reckoned with mean?

: someone who is strong and cannot be ignored When he won his first three matches, the other players realized that he was a force to be reckoned with.

Is it a force to be reckoned with or a force not to be reckoned with?

I understand the phrase "to be reckoned with" means something that must be considered or dealt with, but does "not to be reckoned with" suggest something which is too significant/big to deal with (a force one cannot hope to influence), or something which is too insignificant to deal with (a force of no consequence).