Electricity grids face significant challenges in maintaining reliability amidst increasingly severe and frequent extreme weather events. In addition, grids are under major transformation due to growing electrification across multiple sectors and the decarbonization efforts via increased use of variable renewable energy sources. Expanding transmission infrastructure is one way to meet these challenges. However, planning for transmission expansion is often complicated by multiple planning authorities, jurisdictions, and the need for substantial capital investments. This typically leads authorities to handle transmission investments individually, primarily through intraregional planning, resulting in suboptimal transmission networks. This study explores the potential benefits of cooperative transmission expansion planning, which involves both intraregional and interregional strategies to optimize investments across the entire electricity grid. By utilizing sectoral and economic optimization models, the study evaluates the impacts of varying levels of cooperation among transmission planning regions within the U.S. Western Interconnection for 2019 and 2059 through an iterative investment process. Additionally, it simulates grid operations under future climate conditions and generation portfolio by replaying 2019 heat waves in 2059. The findings suggest that cooperative transmission planning results in lower wholesale electricity prices, fewer energy outages, and reduced greenhouse gas emissions. Although the benefits of collaboration diminish during widespread heat waves, it remains advantageous particularly for regions like the California Independent System Operator, which has significant solar energy capacity. The study highlights the importance of cooperative transmission planning in cost reduction and reliability improvement, advocating for strategic transmission investments to address the challenges posed by future extreme weather events. This research was conducted as part of the Integrated Multisector, Multiscale Modeling (IM3) project funded by the EESM MultiSector Dynamics program area.