As discussed under Jarom 1:8, there is only one occurrence in the earliest text of the phrase “preparation for war”, namely, here in Alma 49:9. In contrast, there are nine instances of the plural “preparations for war” (but eight of these take the verb make, unlike the case here in Alma 49:9). For this single instance of “preparation for war”, the 1906 LDS large-print edition shifted to the plural expression, perhaps unintentionally. That edition never served as a copytext for any subsequent LDS edition. Either reading is theoretically possible, so we follow the earliest extant reading (in 𝓟) for Alma 49:9, “their manner of preparation for war”.
Summary: Maintain in Alma 49:9 the singular preparation in “preparation for war”, the reading of all the earliest (extant) textual sources.