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Dictionary has been consulted but efforts were futile.
Hosni Mubarak rejected the proposals. Washington officials now hope that in the wake of the attack on Egyptian forces in Sinai, Egypt will come on board and fight terror.
."come on board"
The entire world, including the US, Europe, Turkey, Russia and even (and perhaps most of all) the Arab countries are against Iran obtaining nuclear capabilities. There must be a way to get them all on board.
.."to get them all on board"
board - Idioms - by the Free Dictionary, Thesaurus and Encyclopedia.
Have a look at the second definition the first time that "on board" is mentioned. Then there is another segment which has just one definition for it, which is the one you're looking for. Again, in the third segment, there are two definitions and you need the second one.
Remember - if you don't use correct capitalisation, punctuation and spacing, anything you write will be incorrect.