Charleston's Lowcountry charm makes for a deceptively civilized starting point on the long haul to the Rhineland, and after routing through this corridor more times than I can count, I can say plainly: there is no nonstop option, so business class from Charleston to Dusseldorf always means a connection, typically through Atlanta with Delta, Charlotte or Philadelphia with American, or Newark/Washington Dulles with United. Each carrier's transatlantic joint venture partnerships (Delta-Air France/KLM/Virgin Atlantic, American-British Airways/Iberia, United-Lufthansa) mean your onward long-haul leg often flies on a widebody with lie-flat seating, even if the specific aircraft varies by season and demand.
My preference leans toward routing through Atlanta on Delta, since their Delta One suites on the transatlantic widebodies (767 or A330/A350 depending on the exact pairing) offer sliding privacy doors and consistently strong service, and ATL's international terminal makes for one of the smoother connection experiences in the US for a traveler arriving from a regional airport like CHS. United's Polaris product, especially on 767s and 777s out of Newark or Dulles, is another reliable choice, particularly if your Charleston connection timing favors an East Coast hub over Atlanta. American's routing through Charlotte or Philadelphia onto British Airways or Iberia metal is worth considering if you want access to the Club Suite or Iberia's A330 business seat, though I'd double-check aircraft assignments closely since American's partner flying varies by day.
A genuinely useful trick: build in at least a two-hour connection window at your US hub during winter months, since Charleston's regional jet operations are more weather-sensitive than travelers expect, and a missed connection in January snow is far more common than most itineraries account for. January, February, March, October, and November consistently offer the softest business class fares on this Germany-bound corridor, as summer demand into Dusseldorf's trade fair season pushes premium cabins tight. Given how much these fares fluctuate by connection and season, setting up fare alerts for this specific routing is the smartest move you can make before booking.
