Thanks to his familiarity with the Texas
coast as a result of his frequent expeditions
to locate suitable supply depot locations,
Mansfield was appointed head engineer of
General Zachary Taylor’s Northern Army
at the outbreak of the Mexican War in- Accompanying Taylor on the march
across the disputed territory between the
Nueces and Rio Grande Rivers, Mans-
field’s first major assignment was the con-
struction of Fort Texas (later renamed Fort
Brown), a star-shaped, earthen fort oppo-
site Matamoras, near present-day
Brownsville, built to anchor the American
position on the Rio Grande.
Several weeks after the opening skir-
mishes of the war, Taylor marched his main
force to the coast to secure his supply lines,
leaving behind several officers, including
Mansfield, and 500 men to defend the fort.
Mexican gunners opened their assault on
May 3, 1846, and for six days kept the fort
under siege and artillery fire. With no relief
in sight, the Americans boldly took the
offensive. Mansfield led a band of soldiers
out of the fort and blew up Mexican forti-
fications, bolstering morale. Taylor’s sub-
sequent victories at Resaca de la Palma and
Palo Alto forced a Mexican withdrawal.
Mansfield was brevetted major for “gal-
lantry and distinctive service” in defending
the fort. He was not modest about his
achievements, observing in a letter to his
wife, Louisa, that General Taylor owed his
success “more to my opinions before the
battles of Palo Alto & Resaca than to any
other circumstances.”
That September, during the approach to
the city of Monterrey, Taylor’s army came
under artillery fire that halted its advance.
Mansfield, accompanied by a squadron of
dragoons and a company of Texans, led a
small group of engineers forward to con-
duct a reconnaissance of the Mexican
defenses. Such assignments were typical for
the engineers of the time, whose training,
drafting, and map-making skills made them
invaluable to their commanders for con-
ducting intelligence and planning missions.
Mansfield’s field observations were crucial
to the final attack plan, and on September
23, 1846, he personally led a column of vol-
unteers, with a sword in one hand and a
spyglass in the other. Seriously wounded in
the leg, he was brevetted to lieutenant
colonel for “gallant and meritorious con-
duct.” Visited daily by Taylor during his
five-month convalescence, Mansfield recov-
ered sufficiently to act as an adviser during
the Battle of Buena Vista on February 23,- He was brevetted yet again, this time
to colonel, becoming one of a very few offi-
cers who received three brevets during the
war, a list that included Robert E. Lee,
George McClellan, and Joseph Hooker.
In spite of his record, however, Mansfield
remained a captain in the engineers, the
result of reductions in the Army and a
glacially slow system of advancement. On
May 28, 1853, Secretary of War Jefferson
Davis, impressed by Mansfield’s work on
the board of engineers and a witness to
Mansfield’s courage in Mexico, promoted
the 50-year-old captain to colonel and
inspector general of the Army, with respon-
sibility for the vast territory west of the
Mississippi. It was a rare instance of an offi-
cer jumping several ranks in contravention
of the normal seniority rules. General-in-
Chief Winfield Scott, Taylor’s rival during
the Mexican War, opposed the move, view-
ing Mansfield as a “Davis man,” but the
new inspector general proved to be both
effective and independent in his duties.
For the next eight years, Mansfield was
one of the most traveled men in the coun-
try. He toured the New Mexico Territory,
the Division of the Pacific, the Departments
of Texas, Utah, California, and Oregon,
and finally returned to Texas, where he
remained until that state voted for seces-
sion. After a danger-filled journey back to
the capital, Mansfield was placed in com-
mand of the Department of Washington on
April 27, 1861, and three weeks later he
was named one of the first of the newly
authorized brigadier generals in the Regu-
lar Army.
With responsibility for the defense of
Washington and its environs, Mansfield
put his vast expertise on defensive fortifi-
cations to work, supervising the planning
and construction of the entire system of
earthwork installations that protected the
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