The phrase "assembling army" is correct and usable in written English.
It can be used to describe the process of gathering or organizing military forces or troops.
Example: "The general was busy coordinating the logistics of the assembling army, ensuring that all units were in place for the upcoming battle."
Alternatives: "gathering forces" or "mobilizing troops".
When peace with England broke down and Bonaparte was assembling armies along the Channel coast, Ney asked for employment and was given command of the VI Army Corps.
The assembled army proclaimed him king, and his friends adorned him with the diadem.
The Taliban's ability to infiltrate this hastily assembled army reveals the excessive optimism of this policy.
In order to win the support of the Greek city-states, whose resistance to subjugation presented the chief stumbling block to the formation of a Hellenistic monarchy, he announced to his assembled army that all the Greeks should be free, autonomous, and ungarrisoned.
Horne likens it to an assembled army.
The assembled army consisted of contingents of Byzantines, Slavs, Franks, Georgians, Armenians and Christian Arabs.
On 9 May he reviewed the assembled army and gave a speech announcing that the expedition was bound for an unspecified foreign land.
The Mongols retaliated by launching a massive artillery assault on the citadel and when it became apparent that an-Nasir Yusuf was unable to relieve the city with a newly assembled army, the garrison surrendered.
Chinese leader Chiang Kai-shek ultimately decided to defend the city and appointed Tang Shengzhi to command the Nanking Defense Force, a hastily assembled army of local conscripts and the remnants of the Chinese units who had fought in the Battle of Shanghai.
On the evening of April 16, the assembled army, now being referred to as the "Hallie Riflers", escorted Baxter back to the Anthony House, where he set up his headquarters, and from there he began trying to do the state's business once more.
