Grand challenges are complex problems with far reaching societal implications that lack a clear solution. To make progress, diverse communities often coalesce around an ambitious field goal. However, many field initiatives fall short of their initial goals. When fields mobilize for a grand challenge, what inhibits them from realizing their intended ambitions? This is a critical question not only for grand challenges, but also for institutional theory, which tends to focus on field mobilization rather than on how goals are pursued over time. Field scholars often assume stable participants with goals that easily translate into actions, but fields are dynamic and unlikely to comply with these assumptions. With a longitudinal, multi-method study of the nanotechnology field, we examined how five communities mobilized and pursued the grand challenge of creating molecular manufacturing from 1986 to 2005. We identify a key duality of mobilization: the very strategies employed to successfully mobilize diverse participants to support the grand challenge actually helped displace it with less ambitious goals. We develop a grounded theoretical model explaining goal displacement in the context of grand challenges and in so doing contribute a dynamic political understanding of field level strategic action.