It is in the middle and upper Tonkinese region that the bulk of the French forces are found, in which there is only one white regiment - with a few natives, the 5th foreign infantry regiment (5th R.E.I.). All the other formations include a notable proportion of Indochinese:still quite a few in the 9th colonial infantry regiment (9th R.I.C.), more in the "mixed colonial infantry regiments" (R.M.I.C.) and in the 4th colonial artillery regiment ( R.A.C.). In the regiments of Tonkinese skirmishers (R.T.T.), the officers, with a few exceptions, and some of the non-commissioned officers are French, as well as some privates (specialists). The Indochinese workforce comes largely from the delta tonkinese, but one finds, in all the regiments, nationals of ethnic minorities, mountain dwellers from the Sino-Tonkinese border or even, in the 16th R.M.I.C., "rhades", also mountain dwellers, but originating from the plateaus of South Annam, several hundreds of kilometers to the south. Vietnamese from the delta and minorities of all origins are only really comfortable in their region of origin, except for the skirmishers already in service.
The relief of the regions where the operations are taking place is very accentuated and if the Red River, at Lao Kay — 500 kilometers from the sea is only at an altitude of 80 meters, the jagged crests of the Fan Si Pan, between the Red River and the Black River, south of Lao Kay, culminate at more than 3,000 meters. The dense forest in the lower valleys makes it impossible to travel outside the roads and tracks. However, as soon as one rises, the slopes, deforested by the mountaineers, allow movement through thinner stands. Where the trees disappear, the straw grass (the "tran") practically prevents any view and again makes traffic difficult. The roads, with very modest characteristics, are of relatively poor density — 8,318 kilometers for everything
Tonkin (116,000 km’ including 15,000 km’ of fairly well-served delta), of which only 3,859 are paved or paved (1,266) and can be used all year round. Bridges over major rivers are rare and ferries are limited. Many rivers must be forded – in the dry season – or by other means (sampans, rafts). The dry season, in the middle and upper Tonkinese region, ends around mid-April. On the tracks, the horse - saddled or saddled - is commonly used.
Finally, the radio resources of the French army are very insufficient despite the reinforcements from outside in recent months. In most cases, they only allow connections with the outside - Kumming or Calcutta. Rare are the links between bosses and subordinates and even less those between subordinates, except by telephone, with all its disadvantages. The weighted message is still in common use, when you have an airplane. The voice is non-existent.
The Red River Group (G.F.R.), under the command of General Alessandri, straddles the Red River (see map) in the Tong-Son Tay-Viet Tri region with two battalions north of the river (4/19th
R.M.l.C. and 3/5e R.E.l.). The legionnaires are quartered at Cotich — the 3/5th — and at Viet Tri (regimental staff and command staff).
South of the river are the other two battalions of the 5th R.E. 1., a battalion of the 1st R.T.T. and one from the 4th R.T.T. The artillery of the group has three batteries (4th R.A.C.), one of which is towed. Also covered by the G.F.R. :the D.M.C. (colonial motorized detachment), a motorcycle platoon, the aerial observation group (25 Potez at the Tong airfield) and a section of the mounted detachment from the Bac Mai air base (air force personnel who will be used as infantry).
The mission entrusted to the G.F.R. is twofold:to hold the loop of the Red River from Son Tay to Hung Hoa to defend Tong airfield as long as possible, one of the rare large airfields that the Japanese do not control under the terms of the agreements. This mission is only justified in the event of rapid use of the terrain by reinforcements from outside because, to keep it, the G.F.R. would be obliged to engage in a defensive battle backed by two major rivers – the Red River and the Black River – which were very difficult to cross. G.F.R would therefore risk complete destruction, from this first battle, which is incompatible with the second mission entrusted to it:guerrilla actions west of the Black River.
Alerted on the night of the 8 on March 9, General Alessandri meets General Sabattier his arrival in Tong during the same night A major change will be made to the mission of the group which must now hold a bridgehead in the loop of the Red River to allow the crossing of the river to Chien Dong and the Black River Trung Ha. The guerrilla mission remains unchanged. Before joining his command post in Phu Doan, General Sabattier specified that the new system should be in place on 9 May in the afternoon “unless the general situation improves”. The all-clear order issued by Hanoi therefore interrupted the preparations and the troops returned to their cantonments.
On March 9, at 8 p.m., the alert was given by Hanoi and passed on by telephone, from Tong, to all units. The planned system is put in place during the night and the crossing points (Trung Ha and Chien Dong) are set up. On March 10, at daybreak, the combat locations planned for the 2/4th R.T.T. remain unoccupied. The battalion, stationed in Son Dong, was surprised at night, while on the move, and was dispersed. The hole created in the device is somehow plugged.
More serious fact:the garrison of Viet Tri staff of the 5th R.E.I. and elements of protection — was neutralized by surprise by the Japanese on the morning of the 10th. Viet Tri, Son Tay and Phu Tho in the hands of the Japanese, the Chien Dong passage became unusable. The 3/5th R.E.I., although also surprised, around 10 a.m., in its cantonments of Cotich, manages to extricate itself, at the cost of severe losses, and succeeds in crossing the river, at night, to join our forces.