Operation Cobra - Planning

Planning

Further information: Operation Cobra order of battle

The originator of the idea for Operation Cobra remains disputed. According to Montgomery's official biographer, the foundation of Operation Cobra was laid on 13 June. Planning was immensely aided by detailed Ultra Intelligence which supplied up-to-date decodes of communications between German High Command and Hitler's generals. Montgomery's plan at that time called for Bradley's First Army to take Saint-Lô and Coutances and then make two southward thrusts; one from Caumont toward Vire and Mortain, and the other from Saint-Lô toward Villedieu and Avranches. Although pressure was to be kept up along the Cotentin Peninsula towards La Haye-du-Puits and Valognes, the capture of Cherbourg was not an immediate priority. However, with Cherbourg's seizure by Collins' VII Corps on 27 June, Montgomery's initial timetable was soon outdated and the thrust from Caumont was never adopted.

Following the conclusion of Operation Charnwood and the cancellation of First Army's offensive towards Saint-Lô, Montgomery met with Bradley and Dempsey on 10 July to discuss 21st Army Group's next move. During the meeting, Bradley admitted that progress on the western flank was very slow. However, he had been working on plans for a breakout attempt, codenamed Operation Cobra, to be launched by the First Army on 18 July. He presented his ideas to Montgomery, who approved, and the directive that emerged from the meeting made it clear that the overall strategy over the coming days would be to draw enemy attention away from the First Army to the British and Canadian sector; Dempsey was instructed to "go on hitting: drawing the German strength, especially the armour, onto yourself—so as to ease the way for Brad". To accomplish this, Operation Goodwood was planned, and Eisenhower ensured that both operations would have the support of the Allied air forces with their strategic bombers.

On 12 July, Bradley briefed his subordinate commanders on the Cobra plan, which consisted of three phases. The main effort would be under the control of Collins' VII Corps. In the first phase, the breakthrough attack would be conducted by Major General Eddy's 9th and Major General Hobbs' 30th infantry divisions, which would punch a hole in the German tactical zone and then hold the flanks of the penetration while Major General Huebner's 1st Infantry and Major General Brooks' 2nd Armored divisions pushed into the depth of the position until resistance collapsed. The 1st Infantry Division "was to take Marigny, with this objective exploited by a stream of General Watson's 3rd Armored Division armor that would move south toward Coutances." The 2nd Armored Division—part of "Collins' exploitation force" of the 2nd Armored Division in the east of the VII Corps sector and the "1st Infantry Division reinforced by Combat Command B (CCB) of the 3rd Armored division in the west"—would "pass through the 30th Infantry Division sector ... and guard the overall American left flank." If VII Corps' efforts were successful the western German position would become untenable, permitting a relatively easy advance to the southwest end of the bocage to cut off and seize the Brittany peninsula. First Army's intelligence estimated that no German counterattack would occur in the first few days after Cobra's launch, and that if attacks materialized after that date, they would consist of no more than battalion-sized operations.

Cobra was to be a concentrated attack on a 7,000 yd (6,400 m) front, unlike previous American 'broad front' offensives, and would have heavy air support. Fighter-bombers would concentrate on hitting forward German defenses in a 250 yd (230 m) belt immediately south of the Saint-Lô–Periers road, while General Spaatz's heavy bombers would bomb to a depth of 2,500 yd (2,300 m) behind the German main line of resistance. It was anticipated that the physical destruction and shock value of a short, intense preliminary bombardment would greatly weaken the German defense, so in addition to divisional artillery, Army- and Corps-level units would provide support, including nine heavy, five medium, and seven light artillery battalions. Over 1,000 tubes of divisional and corps artillery were committed to the offensive, and approximately 140,000 artillery rounds were allocated to the operation in VII Corps alone, with another 27,000 for VIII Corps.

In an attempt to overcome the mobility constraints of the bocage that had made offensive operations so difficult and costly for both sides, "Rhino" modifications were made to some M4 Sherman and M5A1 Stuart tanks, and M10 tank destroyers, by fitting them with hedge-breaching 'tusks' that were capable of forcing a path through the Norman hedgerows. While German tanks remained restricted to the roads, U.S. armor would now be able to maneuver more freely, although in practice these devices were not as effective as often believed. However, by the eve of Cobra, 60% of First Army's tanks were so equipped. To preserve operational security, Bradley forbade their use until Cobra was launched. In all, 1,269 M4 medium tanks, 694 M5A1 light tanks, and 288 M10 tank destroyers were available.