Now it is certain that mere invasion is not occupation. Invasion is the marching or riding of troops--or the flying of a military air vessel--into enemy country. Occupation is invasion _plus_ taking possession of enemy country for the purpose of holding it, at any rate temporarily. The difference between mere invasion and occupation becomes apparent by the fact that an occupant sets up some kind of administration, whereas the mere invader does not. A small belligerent force can raid enemy territory without establishing any administration, but quickly rush on to some place in the interior for the purpose of reconnoitring, of destroying a bridge or depôt of munitions and provisions, and the like, and quickly withdraw after having realised its purpose.[327] Although it may correctly be asserted that, so long and in so far as such raiding force is in possession of a locality and sets up a temporary administration therein, it occupies this locality, yet it certainly does not occupy the whole territory, and even the occupation of such locality ceases the moment the force withdraws. [Footnote 327: See _Land Warfare_, § 343.] However this may be, as a rule occupation will be coincident with invasion. The troops march into a district, and the moment they get into a village or town--unless they are actually fighting their way--they take possession of the Municipal Offices, the Post Office, the Police Stations, and the like, and assert their authority there. From the military point of view such villages and towns are now "occupied." Article 42 of the Hague Regulations enacts that territory is considered occupied when it is actually placed under the authority of the hostile army, and that such occupation applies only to the territory where that authority is established and in a position to assert itself. This definition of occupation is not at all precise, but it is as precise as a legal definition of such kind of fact as occupation can be. If, as some publicists[328] maintain, only such territory were actually occupied, in which every part is held by a sufficient number of soldiers to enforce immediately and on the very spot the authority of an occupant, an effective occupation of a large territory would be impossible, since then not only in every town, village, and railway station, but also in every isolated habitation and hut the presence of a sufficient number of soldiers would be necessary. Reasonably no other conditions ought to be laid down as necessary to constitute effective occupation in war than those under which in time of peace a Sovereign is able to assert his authority over a territory. What these conditions are is a question of fact which is to be answered according to the merits of the special case. When the legitimate Sovereign is prevented from exercising his powers and the occupant, being able to assert his authority, actually establishes an administration over a territory, it