Hi Virginia
As far as I know Holland hasn't been a part of germany any time.
There has been a french and spanish occupation and the dutch kings and queens had ties to Germany but never was considered to be a part of Germany.
In world war II Holland was occupied by the germans from 1939 till 1945.
For hunderts of years there has been an exchange of people between western Germany(Prusia) and Holland.
Also People from my Family came from Germany to Holland.
A good inetrnet page for geneologie is www.geneologie.pagina.nl were you find a lot of links to archives in holland. The page of the "Rijksarchief" (statearchve) is www.genlias.nl
Goodluck with your research
Joost Sträter