A US-based wine company created a slogan. Later it found a Canadian wine company had already been using the same slogan. Could the U.S. company's used violate copyright or trademark?
Slogans, titles, and other short phrases are generally not subject to copyright protection. They can be subject to trademark protection.
Trademark protection is limited in several ways. It is limited by industry. In this case, according to the question, the two companies are marketing the same sort of product, so the industry limitation would not apply. (A wine slogan might not be protected against use to promote a soda, and surely would not be protected against use to promote a computer brand.) In addition, trademark protection is limited to the geographic market area in which it is used. A slogan or trademark used to promote a grocery chain in upstate New York would not be protected against the use of the same slogan in northern California, for example. Separate countries are usually separate market areas, and in addition, have different trademark laws and trademark registration procedures. (Some countries require registration for any trademark protection to apply. The US does not, although it gives stronger protections to registered trademarks.)
(Trademark protection is also limited to use in trade or commerce, but here both companies seem to be using the slogan in trade.)
If the US company used its slogan to sell wine in Canada, it might be subject to a lawsuit in Canada under Canadian law for infringing the Canadian company's mark. If the Canadian company had been selling wine using the slogan in the US, then it might have grounds for a US suit, under US law, for trademark infringement against the US company.
If the US company was the fist to use the slogan in the US and the Canadian company later started using the slogan in the US, then the US company might have grounds for a suit, as it would have the US rights to that slogan in the wine business. However, if the earlier Canadian use had included advertising that went across the border into the US, that might well reduce the US company's rights, especially if it was relying on use in trade alone, rather than relying on having registered the mark.
If the US company sold and advertised only in the US, and the Canadian company sold and advertised only in Canada, then neither would seem to have valid grounds for a trademark suit against the other.