“Boots on the ground” means “ground troops.” Modern warfare often involves missiles and air support first, followed by “boots on the ground.”
 
“Boots on the grounds” was cited in print in 1990 and “boots on the ground” in 1995. The term “boots on ground” came into common use by 1997.
 
   
Wiktionary: boots on the ground
Noun
boots on the ground
1.(idiomatic, military) The ground forces actually fighting in a war or conflict at the time of speaking, rather than troops not engaged or being transported to the fighting.
 
(Oxford English Dictionary)
boots on the ground: troops who are deployed on active service in a military operation; also in extended use. Cf. on the ground at ground n. 8b; perh. cf. also sense 1d.
[1980   Christian Sci. Monitor 11 Apr. 7/2   Many American strategists now argue that even light, token US land forces—‘getting US combat boots on the ground’, as General [Volney F.] Warner puts it—would signal to an enemy that the US is physically guarding the area.]
1990   Summary World Broadcasts Pt. 4: Middle East, Afr. & Latin Amer. (B.B.C.) 3rd Ser. 22 Jan. ME/0668/B/3   Military analysts said if one took into account the effect of the shortened national service period, the SADF may be left with about 30% to 40% less ‘boots on the grounds [sic]’ than before.
     
2 July 1995, The News Herald (Panama City, FL), “Our fighting men put the pro in professionals” by Jim Wright (Dallas Morning News), pg. 1F, col. 5:
The Marines, dating back to the Revolutionary War and “the shores of Tripoli,” have been able to put boots on the ground in a hurry when trouble boils up somewhere.
 
28 May 1997, Pacific Stars and Stripes, “Air strength clears way for ground troops” by Benjamin S. Lambeth (Los Angeles Times), pg. 12, col. 1:
Despite the determining role played by the allied air campaign in Desert Storm, the Army continues to insist that air power should merely support land operations. It remains wedded to the mantra that “boots on the ground” constitute not just the definitive measure of victory in war but an indispensable condition for achieving such victory.
 
OCLC WorldCat record
The Clash of Visions: The Army argues that it’s “boots on the ground,” not aircraft and precision strike, that matter most
Author: J T Correll
Edition/Format: Article : English
Publication: AIR FORCE MAGAZINE, 80, no. 4, (1997): 2-8
Database: British Library Serials
 
OCLC WorldCat record
“Boots on the ground” : will U.S. landpower be decisive in future conflicts?
Author: Jeffrey Jarkowsky; Army War College (U.S.)
Publisher: Carlisle Barracks, PA : U.S. Army War College, 2002.
Series: USAWC strategy research project. 
Edition/Format: Book : National government publication : English
 
OCLC WorldCat record
Boots on the ground : a month with the 82nd Airborne in the battle for Iraq
Author: Karl Zinsmeister
Publisher: New York : Truman Talley Books/St. Martin’s Press, 2003.
Edition/Format: Book : Biography : English : 1st ed
 
OCLC WorldCat record 
Call to duty : boots on the ground.
Author: United States. Department of the Army. Office of the Chief of Public Affairs.
Publisher: [Arlington, VA] : U.S. Army, Army Strategic Planning Division, OCPA, [2006]
Edition/Format: DVD video : National government publication : English
 
OCLC WorldCat record
Boots on the ground by dusk : my tribute to Pat Tillman
Author: Mary Tillman; Narda Zacchino
Publisher: New York, NY : Modern Times, ©2008.
Edition/Format: Book : Biography : English