Operation Thayer I — The Kim Son Valley Search-and-Destroy (Sep–Oct 1966)

Operation Thayer I — the Kim Son Valley search-and-destroy

Context page — operation level. Built from the 1st Cavalry Division's own Operational Report — Lessons Learned for the quarter ending 22 November 1966, which narrates Thayer I at the division level, with a corroborating Staff Historian entry from the following quarter's report. It frames the operation that D Company, 2/8 Cavalry fought through in September 1966 — the operation in which 2LT Michael Derosier was killed near Bong Son on 19 September and LTC Tackaberry earned his second Distinguished Service Cross. Division- and brigade-level movements are well documented; D Company's hour-by-hour positions await the battalion staff journal.

What it was

Operation Thayer I (13 September – 1 October 1966) was a 1st Cavalry Division search-and-destroy operation in Binh Dinh Province, launched in response to an enemy build-up in the Kim Son ("Crow's Foot") Valley (vicinity grid BR1480). It was the first phase of a long sequence that ran through that autumn and winter: Thayer I → Irving (2–24 Oct) → Thayer II (24 Oct – 12 Feb 1967), all on the same Binh Dinh ground.

The division's scheme, in its own words, was to attack the enemy bases and facilities with two brigades while squeezing his forces eastward toward the seacoast. In the initial assault, five battalions were air-assaulted onto high-ground landing zones around the Kim Son, then worked the ridge lines and fingers down to the valley floors. Contact was sporadic, but the division met intense anti-aircraft fire throughout the area of operations. On 20 September three battalions were air-assaulted into the valley east of the Kim Son (vic BR8580) to exploit reports of an enemy build-up there. On 23 September the enemy struck back at the division's HAMMOND logistics base with mortars, recoilless rifles, and small arms, causing light damage and casualties. When Thayer I terminated on 1 October, its logistics elements were already positioned to roll straight into Operation Irving the next day.

Where 2/8 Cavalry — and D Company — fit

2/8 Cavalry fought Thayer I as a 1st Brigade battalion under LTC Thomas H. Tackaberry, who had taken command on 30 July 1966. The single best-documented D Company moment of the operation is 19 September 1966, when a fifteen-man D Company patrol — 2LT Michael Derosier's platoon — was caught by a reinforced North Vietnamese company and pinned down. Derosier, the platoon leader, was killed; Tackaberry landed his unarmed command helicopter beside the action, ran through the fire, took personal command, and led the assault that broke the encirclement — the action of his second Distinguished Service Cross. That day is documented in full on the 19 September 1966 contact page.

A note on location: Tackaberry's DSC citation places the action "near Bong Son," while the division Staff Historian's record — prepared with retired BG S.L.A. Marshall, who documented 1st Cavalry actions in this period — logs the 2/8 Cavalry action of 19 September 1966 in the "506 Valley" under Thayer I. Both lie in the Bong Son / Kim Son area of Binh Dinh; the archive keeps both labels and flags the discrepancy rather than forcing a single grid.

What's still open

The division ORLL is written at division and brigade level, so D Company's specific daily positions within Thayer I — and the exact grid of the 19 September action — are not in it. Those await the 2/8 Cav daily staff journal (NARA RG 472) and the Thayer I after-action report.

Help us complete the record

If you served with D Company, 2/8 Cavalry during Operation Thayer I, or knew 2LT Michael Derosier, please reach us through the contribute form.