[Alabamascientist (OP)]

The Dutch started building dikes hundreds of years ago to prevent flooding.  Did the sea level rise at that time and cause the flooding?  If so, why did the sea level rise?  Global warming?

George "The Alabama Scientist"

[eric l]

The dikes were build mainly to protect areas that where frequently flooded in the first place, and to gain land on the see in a later stage.  So a lot of Holland was below sea level - even without this level rising - and many of those famous windmills were build to pump up water from a low level canal to a higher level and so on. 
Even if from the 19th century on the pumping stations were powered by steam or diesel engines rather than windmills, the Dutch word for them has remained "gemaal", which could be explained as "the action or working of a mill" (or "the product of milling").
The efforts in the 20th century increased the dry surface of the Netherlands with more than 10 % (actually the complete province of Flevoland).

[another_someone]

The propensity for flooding in the absence of active pumping was a strategic factor the Dutch freedom fighters used to great effect in their war of independence against the Spanish in the 16th century (and the 16th century would probably have been the point the sea was at its lowest for several millenia).

[Alabamascientist (OP)]

So---they were actually creating "new land" rather than recovering previously flooded land?

George

[eric l]

Actually, both happened (reclaiming land and "creating" land). 

In the "Low Countries" (which extend beyond what is now the kingdom of the Netherlands), there has been a constant struggle from prehistoric times onwards.  There have been "transgressions" when the sea invaded the land, followed by periods when land was reclaimed.

The nationally organized building of dikes, followed by evacuating the water from the areas between them, is documented from the 14th century onwards, and has been constantly intensified.  It probably started much earlier, as local efforts, but in those days the water from the polders could only be evacuated at low tide.

Man has repeatedly used the flooding of such areas for defensive purposes, mainly in the Dutch wars of Independence against Spain (16th and 17th century) but even as late as the beginning of the first World War.  The Belgian army retreated behind the river Yser, where they had a railroad embankment more or less parallel to the river.  They plugged all passages (creeks, farmroads...) in that embankment and opened the locks at high tide, allowing the sea to flow inland.  Of course, this was not the only thing that stopped the Germans, but it did save the Channel ports on the French side.