Usually substantivized adjectives carry the idea of plurality with the sole exclusion:

The deceased – покойные (покойники)

The deceased – покойный (покойник)

Thus, you would expect “As Ukraine's military step up their strikes”, but it is not the case:

As Ukraine's military steps up its strikes on Kherson, hinting at a new offensive to recapture the region, there is another force working alongside. They are Ukraine's shadow army, a network of agents and informers who operate behind enemy lines.

https://www.bbc.com/news/world-europe-62333795