The Island of Ysselmonde and Alblasserwaard
The Dutch force at the Alblasserwaard had suffered continuously from German Luftwaffe raids. The Germans apparently feared the Dutch occupation of the eastern banks of the river Noord, since the tactical forces of the Luftwaffe seem to have been very much focussed on these positions during the May-war.
A British-German airbattle
Also in the early morning hours of the 13th between 0430 and 0700 hours Stuka's assaulted the Dutch artillery and infantry positions around Alblasserdam. But this time they would meet some opposition in the sky.
In the evening of the 12th the RAF had planned a strafing mission of six Defiants of 264th Squadron and six Spitfires of 66th Squadron along the Dutch coast. As this combined squadron approached the Dutch coast they were treated on Dutch anti-aircraft fire; fortunately without sustaining any damage. They proceeded south towards the Hague and Rotterdam. Then they spotted the Luftwaffe Stuka's south-east of Rotterdam.
The Stuka's  were of the 12th squadron of LG1 [12e Staffel, Lehr Geschwader 1]. As they dove towards Dutch positions at Alblasserdam suddenly the twelve British fighters appeared. A huge dogfight unfolded when the Germans got fighter back-up after a red flare was launched from the leading Stuka. The arrival of Messerschmitt Bf-109 [24-27 of JG26 - Jagdgeschwader] fighters meant that the kettle was put on the fire. The dogfight gradually expanded to the skies over Brabant.
After the massive clash between the two forces [together about 40-45 planes] four German Stuka's [the RAF claimed seven - only four wreckages were identified], two Bf-109's and five of the vulnerable British Defiant's had been destroyed - killing most of the crews. It is a miracle that the British airmen managed to escape total annihilation after being confronted with the overwhelming German force and even managed to take out more enemy planes than they suffered own losses.
The 9th Tank Division in arrival
In the meantime Dutch artillery batteries [two batteries of 7,5 cm, two of 10,5 cm] had started pounding the German bridgehead at the western banks of the Noord. Although during the Stuka raids the guns were temporarily left by the hiding crews, the artillery units did not give way. Hundreds of grenades landed on the German positions. Surprisingly however the artillery was not ordered to shell the German armoured columns crossing the traffic bridge at Zwijndrecht. This appeared still to be a result of a highly questionable order of the Dordrecht garrison commander Mussert who had already declared on the first day of the war that the un-evacuated cities of Dordrecht and Zwijndrecht were not to be shelled ... New initiatives and visions of local commanders with more common sense seemed to have been absent. The German tanks were therefore not hindered or harmed in their advance!
The aforementioned German tank forces crossed the Zwijndrecht bridge at the 13th in the afternoon. They proceeded in the direction of the south of Rotterdam. The first offensive action the tanks undertook after crossing the bridge was at Barendrecht. Here a Dutch assault had been rejected the day before and Student considered it necessary to secure the southern land head of the bridge by using the force of some medium tanks.
On the Dutch side [Group Kil] about three sections [75 men] had dug themselves in. They were supported by 8 machine guns and 2 anti-tank guns. These forces were partially of the same battalion that had occupied Wieldrecht, the 3rd Border Infantry Battalion. Along the river [Oude Maas] more Dutch units occupied several strongholds. They exchanged fire with the Germans and even some minor Dutch crossings [by boat] had been executed. All these actions ended in retreats.
At 1900 hours the Germans, opposing the Dutch troops at the Barendrecht bridge, suddenly opened a massive fire with machineguns, mortars and 2 cm tank guns. Simultaneously one leading Pz.III and three Pz. II tanks appeared on the bridge. Some Dutch defenders gave way but the anti-tank gun crews stood firm. Three of the four tanks were destroyed in no-time. The escaping crews were suppressed by fire from Dutch machine guns. Four Germans perished. Three tanks were total write-offs. During this engagement the Dutch lost two men.
Although the assault had been rejected, the rumour spread round along the river forces that the bridge had fallen in German hands and that a German assault to their positions from the land side was imminent. As a result of this panic the battalion commander [3rd Border Infantry] ordered his troops to leave their defences and move back to the south. The result of this very unfortunate rumour and subsequent order turned out to be epidemic. The commander of the Group Kil was informed of the loss of the bridge and the retreat of his troops. He immediately realised that the lost bridge would facilitate the Germans to assault his units with tanks and armoured cars to which hardly any defence could be made. As such he ordered his entire Group [taking with him the adjacent Group Spui] to clear the Island of Hoekse Waard and retreat cross the Spui.
The subject Major - who had ordered the retreat of the 3rd Border Infantry Battalion - was prosecuted after the war, but relieved of all charges. In the end the retreat did not contribute to the Dutch defeat. The Germans, although offered the free access to the Island of Hoekse Waard, were not very interested in dealing with the Dutch forces south of Rotterdam. They would leave that task to troops from the rear. Rotterdam itself was their prime target.
The petrol harbour
The only target on the Island of Ysselmonde that did tickle German officers was the BPM [later known as Dutch Shell] installation. An extended petrol vessel parc with precious installations. This was situated in the north-western corner of the Island of Ysselmonde and had been occupied [and successfully defended] by a company of infantry, supported by some heavy machine guns. At this point a major crude oil offload and storage facility was situated. The Germans were in particular interested in these facilities and for that reason they were very careful during their operations against the Dutch defenders overthere. No heavy weapons were allowed, let alone tanks and guns.
British engineers had been shipped in from England to see to thorough destruction of these facilities once the Dutch armed forces would fail their defences. At the 13th a new attempt was made by a limited German task force [probably air landing troops of the 72nd Regiment] to overrun the Dutch. The defending force did however withstand the German attack, although considerable losses were suffered [on both sides]. Hereafter the Dutch local commander decided to retreat to the north of the harbour after he had given protection to the Dutch and British engineers who executed their demolishing work. After some intensive preparation the vessels loaded with crude oil blazed and poured out thick black smoke. It would be the prelude to much more destruction in the Rotterdam area ...
With this last event the Dutch opposition south of Rotterdam had ended. It was then up to the defenders of the town itself.