I am using SourceTree as a Git client in my windows 7. I faced with a conflict and in order to resolve I click on Launch external merge tool as suggested. A window pops up very shortly and nothing happens then after!
I just tried it on my Mac OS and it shows external merge tool as expected.
under Tools -> Options -> Diff -> External Diff / Merge dropdown box, System Default is selected.
I expect to see the diff merge tool but it never shows up in my windows, anyone know what could be the reason?
Do you have a diff tool installed on your system?
In Mac, when you install Xcode you get a diff tool named FileMerge, simple but sufficient in most of the cases. You can try WinMerge for Windows, I've used it in the past and it's pretty nice, and open source.
You can also try the P4 Merge tool from Perforce which is offered for free.
Both options can be selected as a Diff/Merge Tool in SourceTree.