Amestris has been at war with its southern neighbor Aerugo for a longer time than its ongoing border war with its western neighbor Creta. The war is mostly in Amestris' favor because of its sheer military strength. Aerugo is putting up stiff resistance however, and is constantly using most of its army to continue to hold off Amestris. The war has had a destructive impact on Aerugo's territory.
The war began because Amestris took the town of Fotset as well as a strip of Aerugo's borders. This was seen as an insult at first by the Aerugo Royal Family. Aerugo then brought in heavy amounts of men, not knowing that it was playing exactly into Amestris' plan. Amestris slaughtered the Aerugo forces easily with their superior military technology, sheer amount of Men, and powerful alchemy. All of South City quickly turned into an uproar and South Headquarters sent out most of its forces. These forces ripped through Aerugo's and charged deeper into Aerugo's lands. Aerugo sent over half of their military to stop the purge and was successful in pushing Amestris back to an Aerugo border town. Because of this, Southern Amestris begin losing soldiers, population, and influence in the government. Southern Amestris stopped helping to exterminate Ishvalans in order to focus on the Aerugo war by sending in its alchemists. This was ineffective, and Amestris' forces were slowly pushed back into Fotset.
Currently, Amestris' Southern forces are fighting hard against Aerugo and are being reinforced by Central city with tanks and troops alike. The East is managing to spare a small amount of troops as well. Amestris is slowly gaining ground because their military technology, alchemy, elite soldiers, and superior numbers. Once Aerugo has been completely pushed back into its land, Amestris plans on attempting to conquer more of the country.