The indecision of OKL over what to do was reflected in shifts in "Luftwaffe" strategy. The doctrine of concentrated close air support of the army at the battlefront succeeded against Poland, Denmark and Norway, the Low Countries and France but incurred significant losses. The "Luftwaffe" had to build or repair bases in the conquered territories, and rebuild their strength. In June 1940 they began regular armed reconnaissance flights and sporadic "Störangriffe", nuisance raids of one or a few bombers by day and night. These gave crews practice in navigation and avoiding air defences and set off air raid alarms which disturbed civilian morale. Similar nuisance raids continued throughout the battle, into late 1940. Scattered naval mine-laying sorties began at the outset and increased gradually over the battle period.