those under someone

serve under someone

declare war on someone

draw a bead on (someone or something)

give someone a blanket party

have (someone) at bay

not be able to get (someone) on the comms

throw someone in the meat grinder

blaze at someone

hold someone at bayonet point

lie prone on someone's forefront

pose as someone

raise someone on the net

arouse someone by trumpet

draw someone into ambush

foot march under packs

hand-held under-barrel grenade launcher

restricted area (under military jurisdiction)

surveillance, under surveillance

under armor auxiliary power unit (UAAPU)

under arms

under barrel grenade launcher (UBGL)

under canvas

under command of

under cover of

under fire

under hatches

Under Secretary of Defense (USD)

Under Secretary of Defense for Acquisition, Technology, and Logistics (USD AT&L)

Under Secretary of Defense for Intelligence USD(I)

under-strength unit

under-vehicle improvised explosive device (UVIED)

nap-of-the-earth (NOE) (ground-hugging / terrain masking / flying under the radar / hedgehopping)