Poul Anderson 
Ensign Flandry [EPUB ebook] 
A Flandry Book

Dominic Flandry had a great future ahead of him as saviour of the civilised universe. In later years his talent for swift, decisive action would give him an intergalactic reputation. But at the age of nineteen and straight out of naval academy, he was just another raw ensign.The mighty Merseian Empire had sworn to wipe the Earth from the face of the universe. The attack had already been launched, but no one knew how or where the ravening power of the savage green skinned aliens would strike. Only Ensign Flandry had the answer, in the form of a code which he might — or might not — be able to decipher.And so the Merseians were coming after Flandry with every weapon in their terrible arsenal. And just to make things worse, Earth’s own armadas were after him too — for desertion, high treason and other assorted crimes. Even for a future saviour, times were looking pretty tough.