WORF: Romulans.
PICARD: That's the assumption.
RIKER: There's been no direct contact with the Romulans since the Tomed Incident.
PICARD: The question are, why now? What's their objective? For
fifty years there's
barely a whisper out of them, and now for no apparent reason they seem to be back with a roar.
RIKER: Everything we know about them is based on rumour or conjecture.
…
DATA: Since we have
no contact with the Romulans for
fifty three years, seven months, eighteen days,
[2311] we must consider that the information we do have, is out of date.
…
RIKER: Amazing. I never thought I would ever see a Romulan ship. Not this close.
LAFORGE: The last time we encountered them was decades ago. Cost thousands of lives.
Star Trek: The Next Generation "The Neutral Zone"
2311:
A conflict between the Federation and the Romulan Star Empire, known as the Tomed Incident takes place, the outcome of which led to the Treaty of Algeron. The treaty redefined the Romulan Neutral Zone, and as well, ended all Federation research into cloaking technology. Afterward, the Romulans go into a period of complete isolation. (TNG: "The Neutral Zone", "The Pegasus"; ENT: "These Are the Voyages...")
Source:
http://memory-alpha.wikia.com/wiki/2311