Xbox 360 Slim + Xbox Live = Internet Required (No Exceptions)

Yes, absolutely. The Xbox 360 S ("Slim") has no offline-only Xbox Live mode.

Why

• Xbox Live is a network service — authentication, matchmaking, marketplace, cloud saves, parties, messages all traverse Microsoft's servers.
• No local emulation: Unlike LAN/System Link (direct console-to-console), Live requires internet routing.
• Digital Licenses: Even single-player digital games phone home every 30 days (Home Xbox) or every launch (non-Home).

Minimum Connection

• 512 Kbps up/down — functional for matchmaking, voice, small downloads.
• 3+ Mbps — comfortable for 720p/1080p game downloads, party chat.
• NAT Open — UPnP enabled on router; otherwise "Strict" blocks many hosts.

No Internet? You Can Still:

• Play disc-based single-player games (on Home Xbox).
• Watch DVD (360 S has no Blu-ray; original 360 none either).
• Use local System Link (Ethernet crossover / switch) for supported titles (Halo 3, CoD, Gears).

Bottom line: No internet = no Xbox Live. Plan accordingly.