English serves as the primary operational language of NATO, with proficiency assessed through the STANAG 6001 examination. This study analyses 240 authentic texts produced during the STANAG 6001 Level 2 writing examination at the University of Defence using corpus-based error analysis. Results indicate that failure to achieve Level 2 is mainly caused by systematic morphosyntactic errors and omission of obligatory language elements. Level 2 texts demonstrate greater grammatical accuracy and complexity, with implications for targeted English language instruction in military higher education.