Complex sentence: Fighting in Rotterdam had been going on since the first day of the campaign, when German soldiers in seaplanes landed on the Maas river and captured several bridges intact; however the Germans were uneasy about risking a tank attack on the city and feared heavy casualties.
Split into simpler sentences: Fighting in Rotterdam had taken place since the first day of the campaign, when German infantrymen in seaplanes landed on the Maas river and captured several bridges intact. Nevertheless, the Germans were uneasy about risking a tank attack on the city and feared heavy casualties.