British Command the first intimation of what was happening.
Here is one of them:

_Date_: 31.8.1914. _Pilot_: Lieutenant A. E. Borton.
_Observer_: Captain E. W. Furse.

_Hour at which reconnaissance commenced_: 9.20 a.m.

_Time._ _Place._ _Observation._

9.55 Villeneuve. Motor T.

9.58 E. Roberval. "

10.2 At Station N. of Motor T. halted clear of road.
Verberie.

10.6 La Croix. Cavalry and transport much opened
out, head La Croix.

10.13 Compiegne. Clear.

10:20 Chevincourt. Cavalry about 1 Bde. moving towards
Thourotte. Head near that
place. More cavalry and guns
following across fields. Bivouac at
Chevincourt. Transport coming
in from Marest road 10.25.

10.26 Mareuil. Cavalry column still continues--opened
out--new column trotting S.

10.27 Lassigny. Artillery just S. of town moving
south. Column ended just S. of
Lassigny (1/2 mile). Another column,
tail Lassigny, was moving towards Thiescourt.
Also mounted troops. About one mile N. of
Lassigny mounted troops in bivouac
and on road from Roye, stretching
right up to Roye. Columns included
guns and motor transport.
Parked transport just N. of Roye.

10.50 Roye. Three Batteries halted in field 1-1/2 m.
S.E. of Roye. Besides the column
stretching south to Lassigny,
there was another column stretching
S.E. on Noyon road. German
aeroplane on ground S. of Roye.

10.55 Conchy. Infantry and guns moving due east
through Conchy.
Orvillers. Few troops in village.

11.0 Ressons. Went up road east of railway. This
was clear.

11.3 Margny. Squadrons moving S.E.

11.7