Why sustainment mattered as much as the landing

What was more difficult: the D-Day assault or keeping the invasion force supplied?

Both were dangerous, but sustaining five divisions before a port was secured was the larger staff and logistics challenge, which is why Mulberry harbours mattered.

The immediate assault was terrifying, but the continuing problem was building and maintaining combat power ashore. Mulberry harbours were being established while the beaches were still insecure because the supply flow could not wait for a captured port. The temporary harbour system allowed the invasion to expand beyond its first waves.