In this article, we take issue with the claim by Sunstein and others that online discussion takes place in echo chambers, and suggest that the dynamics of online debates could be more aptly described by the logic of ‘trench warfare’, in which opinions are reinforced through contradiction as well as confirmation. We use a unique online survey and an experimental approach to investigate and test echo chamber and trench warfare dynamics in online debates. The results show that people do indeed claim to discuss with those who hold opposite views from themselves. Furthermore, our survey experiments suggest that both confirming and contradicting arguments have similar effects on attitude reinforcement. Together, this indicates that both echo chamber and trench warfare dynamics – a situation where attitudes are reinforced through both confirmation and disconfirmation biases – characterize online debates. However, we also find that two-sided neutral arguments have weaker effects on reinforcement than one-sided confirming and contradicting arguments, suggesting that online debates could contribute to collective learning and qualification of arguments.