Merge

From AACWWiki

Units may be merged at any time by dragging and dropping their tab onto the tab of the stack you want it to merge with. The same can be achieved by dragging the stack (on the game-map) onto the stack you want it to be merged with. Ultimately, units may merge as a result of the resolution phase when you try to merge stacks that come from different regions as a result of an intercept order called onto a friendly stack.