The phrase "army assembling" is correct and usable in written English.
It can be used to describe the process of military personnel gathering or organizing for a specific purpose, such as preparation for deployment or training exercises.
Example: "The army assembling at the base was a clear sign that preparations for the upcoming exercise were underway."
Alternatives: "military gathering" or "troops mobilizing".
He invaded Yorkshire (1319) with Thomas Randolph, earl of Moray, defeating an English army assembled at Myton-upon-Swale.
The directors, who wanted to launch an invasion of the British Isles, appointed Bonaparte to command the army assembled for this purpose along the English Channel.
Theodahad, deposed by the threatened Gothic army assembled in the Pontine marshes and replaced by Witigis, fled toward Ravenna, but he was captured and killed by a Goth on the Via Flaminia.
In 1794 his political career was interrupted while he commanded the army assembled to put down the Whiskey Rebellion, an uprising of farmers resisting the federal whiskey tax, in western Pennsylvania.
Soon colonial leaders deemed the Narragansetts in violation of a series of peace treaties, leading the United Colonies of Massachusetts Bay, Plymouth, and Connecticut to amass the largest colonial army assembled to date in New England, consisting of 1,000 militia and some 150 Indian allies.
William's army assembled during the summer while an invasion fleet in Normandy was constructed.
In September, Benedict Arnold led 1,100 troops on an expedition through the wilderness of Maine, which was drawn from the army assembled outside Boston.
His army assembled at Nikli, crossed the Isthmus of Corinth, and at the pass of Mount Karydi, on the way from Megara to Thebes, his army decisively defeated the coalition army.
The army (assembled by the Duke's brother, General Charles Churchill) consisted of 66 squadrons, 31 battalions and 38 guns and mortars totalling 21,000 men (16,000 of whom were English troops).
Perhaps a lot of the threats to the realm of international order won't be foreign armies assembling on the battlefield, but will arise from the various barbarians at the gate.
