The Expedition of Alqammah bin Mujazziz was sent to deal with a group of Abyssinian (Ethiopians) pirates. They were criminals and had to be dealt with in order for the area to be safe.
Kitab al-tabaqat al-Kabir – Ibn Sa’d:
“Then (occurred) the sariyyah of ‘Alqamah Ibn Mujazziz al- Mudliji against al-Habashah in the month of Rabi’ al-Akhar… They (narrators) said: A report reached the Apostle of Allah, may Allah bless him, that the people of Juddah had seen the people of al-Habashah (Abyssinia). Thereupon he sent ‘Alqamah lbn Mujazziz at the head of three hundred persons. He reached an island in the sea. When the (tide) rose in front of them they ran away from it. When it reached, some people hastened to go to their families and he permitted them. …” 
Dr. Shawqi Abu Khalil:
“THE EXPEDITION OF ALQAMAH IBN MUJAZZIZ AL-MUDLIJI TO JEDDAH IN RABI’UTH-THANI IN 9 AH.
Three hundred men went with him, to repel a group who had come by sea from Abyssinia.” 
Safiur Rahman Mubarakpuri:
“The three hundred men expedition of Alqamah bin Mujazziz Al-Mudlaji to Jeddah shores in Rabi Al-Akhir. This expedition was dispatched to fight against some men from (Al-Habasha) Abyssinia (Ethiopia), who gathered together near the shores of Jeddah and exercised acts of piracy against the Makkans. Therefore he crossed the sea till he got to an Island. But as soon as the pirates had learned of Muslims arrival, they fled.” 
“An expedition of three hundred led by AlQamah bin Mujazziz went to shores of Jeddah in Rabi al-Akhir to bring an end to piracy. The criminals fled.” 
Don’t forget to follow Discover The Truth on Facebook and Twitter. PLEASE help spread the word by sharing our articles on your favourite social networks. Thank You!
 Kitab al-tabaqat al-kabir, By Ibn Sa’d,volume 2, page 201 – 202
 Atlas Al-sirah Al-Nabawiyah – Atlas On The Prophet’sBiography Places, Nations, Landmarks, By Dr. Shawqi Abu Khalil, Page 235
 Sealed Nectar – Ar-Raheeq Al-Makhtum, by Safiur Rahman Mubarakpuri, page 187
 Notes On Entering Deen Completely: Success, Ummah, Renewal By Talib Jaleel, page 512