Complex sentence: The plans in the Napoleonic era had called for a link to the Scheldt at Breskens, but by the time the canal was only opened to traffic in 1856, the link with the sea had moved east to Sluis.
Split into simpler sentences: The plans in the Napoleonic era had called for a link to the Scheldt at Breskens. Half a century later the canal opened to traffic in 1856, and the link with the sea had moved east to Sluis.