This is clearly the word REINFORCEMENTS and, using the letters thus found, the rest of the line becomes AMMUNITIONAND. We have then the following letters determined:

Arbitrary lettersABCDEFGHIJKLM
Plain TextREINFOCMTSAUD

If these be substituted we have for the message:

REINFORCEMENTS AMMUNITION AND RATIONS MUST ARRI_E _EFORE T_E FIFTEENT_ OR _E CANNOT _O_D OUT_.

From this the remainder of the letters are determined:

Arbitrary lettersNOPQRS
Plain textVBHWLX

Now let us substitute the two-letter groups for the arbitrary letters:

Arbitrary lettersKOGMBEPCRHDFAJILNQS
Two-letter groupsGGRGAGNGTGGRARNRGARAAANARNANNNTNGTRTAT
Plain textABCDEFHILMNORSTUVWX

It is evident that the cipher was prepared with the letters of the word GRANT chosen by means of a square of this kind: