I examine two alternative strategies that an environmental group can embark when interacting with a firm. The first one which is already extensively discussed in the literature is when the group campaigns against the firm. The second one which has not been modelled in the literature yet is when the group collaborates with the firm (green alliance) by sharing its know-how in order to reduce the implementation cost of the cleaner technology. The main result of the paper argues that for higher taxation the conflict scenario is more likely to happen, implying that collaboration and a more stringent environmental policy are substitutes. This identifies a previously unexamined and possibly adverse effect of public policy on environmental quality because it weakens the desirable impact of the pollution tax on emission intensity since the latter is higher under conflict and reinforces the, already negative, effect of environmental policy on output. Due to the complexity of the problem, I undertake numerical examples to calculate the optimal tax that maximises Social Welfare and I find that tax under pure conflict --when conflict is the only option for the environmentalists- is higher than in the case where the group can choose to either act against or join forces with the firm, indicating that a more stringent environmental policy is needed in the first scenario.