Being harassed by neighbors. Landlord knows and does nothing. Is he liable?

Are the neighbors also tenants of the same landlord? If not, the landlord has no control over the neighbors, and would therefore not have any legal duty to stop the neighbors from harassing you. If the neighbors are the tenants of the same landlord, then perhaps. Under California law, all leases have the implied covenant of "quiet enjoyment". (California Civil Code, § 1927). The landlord (and management company) has the duty to preserve the quiet enjoyment of all tenants. (Davis v. Gomez (1989) 207 Cal.App.3d 1401, 1404.) What you want to establish is a "constructive eviction" theory. The concept of a “constructive eviction” exists under the rubric of a breach of the covenant of quiet enjoyment that is implied in every rental agreement. (Stoiber v Honeychuck (1980) 101 Cal.App.3d 903, 925–926.) Substantial interference is required to establish a breach of quiet enjoyment. An interference by the landlord "by which the tenant is deprived of the beneficial enjoyment of the premises amounts to a constructive eviction if the tenant so elects and surrenders possession, and the tenant will not be liable for rentals for the portion of the term following his eviction." (Kulawitz v. Pacific Paper Co. (1944) 25 Cal.2d 664, 670.) Minor inconveniences and annoyances are not actionable breaches of the implied covenant of quiet enjoyment. To be actionable, the landlord's act or omission must substantially interfere with a tenant's right to use and enjoy the premises for the purposes contemplated by the tenancy. (Petroleum Collections Inc. v. Swords (1975) 48 Cal.App.3d 841, 846.)

As noted, the landlord has no duty to his own tenants if the harassment is caused by a neighbor who lives in a different building. And as for being "forced" out, that's a subjective determination. You don't provide any detail about what kind of harassment you're referrig to, but obviously not everyone responds to things in the same way, so not everyone would feel forced to move by the same irritation. Is this bad enough to call the police? If so, call the policie. Is it more like bullying? Then it's unlikely that your landlord is going to get involved.