You don't necessarily need to register a trademark in order to defend it (although you do need to defend it once it's registered unless you want to risk using it).

From Wikipedia:

> In the United States, neither federal nor state registration is required to obtain rights in a trademark. An unregistered mark may still receive common law trademark rights. Those rights, for example, may extend to its area of influence—usually delineated by geography. As such, multiple parties may simultaneously use a mark throughout the country or even state. An unregistered mark may also be protected under the federal "Lanham Act" (15 USC § 1125) prohibition against commercial misrepresentation of source or origins of goods. Unregistered marks are also protectable in the United States under Lanham Act §43(a).

https://en.wikipedia.org/wiki/Unregistered_trade_mark