Not necessary.
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PeaceblinkfriendThe Australian Government's overseas aid program aims to assist developing countries (to) reduce poverty and achieve sustainable development, in line with Australia's national interest. http://www.ausaid.gov.au/makediff/default.cfm
YankeeI'd use 'in' (rather than 'to' or no preposition):
... assist developing countries in reducing poverty and achieving sustainable development, ...
If you used 'help' rather than 'assist', I would accept either 'to' or nothing -- i.e. help