are military messages in all capitals?
The English language contains 26 letters and 10 numbers, plus about 5 special characters (space, period, comma, end of line). This is a total of 42 characters needed.
If you were to add in the capability to distinguish between upper-case and lower-case letters; you would need 52 letters, 10 numbers, and 5 special characters, for a total of 67.
A 6-bit system (the minimal capable of carrying an English transmission) is capable of holding 64 characters, while a 7-bit system, can hold 128.
The average teletype circuit is capable of delivering a data-rate of 40~ bits a second. The following message:
MSGID/OPREP-3PEQ/USS BULLET/001 B//
GENTEXT/DAMAGES/TWO TLAM-N MISSILES DAMAGED, WARHEADS REMOVED BY TERRORISTS//
GENTEXT/INCIDENT CAUSE AND DETAILS/USS BULLETT BOARDED BY 10 ARMED TERORISTS, WHO REMOVED WARHEADS FROM 2 TLAM-N MISSILES. THREE CREW MEMBERS WOUNDED IN ATTACK. TERRORISTS DEPARTED BY HELICOPTER//
GENTEXT/SEIZURE INFORMATION/HELICOPTER HEADED FOR MEXICAN COAST. MEXICAN AUTHORITIES NOTIFIED. SHORE-BASED MILITARY AND CIVILIAN AUTHORITIES NOTIFIED. AIR SEARCH IN PROGRESS//
GENTEXT/PUBLIC AND POLITICAL IMPLICATIONS/MEXICAN GOVERNMENT NOTIFIED AND PROMISE FULL COOPERATION//
GENTEXT/ASSISTANCE REQUIRED/REQUEST MEDEVAC OR INJURED CREW MEMBERS//
RMKS/ADDITIONAL REPORT WILL FOLLOW//
Has a total character count of around a thousand.
With 6-bit (all caps) encoding, it would require 150 seconds to transmit over our notational teletype circuit; while a 7-bit (upper/lowercase) encoded message would require 175 seconds.
Multiply this by the number of messages which pass through a typical high level military command post each day, and you can see why lower case letters are not used.
Additionally, capital letters are easier to distinguish from one another when you have low resolution prints or copies of the original message.