That's the traditional islamic practice called hudna, calling a "truce" only to use the time gained by the "truce" to come back even stronger:
"Hudna has a distinct meaning to Islamic fundamentalists, well-versed in their history: The prophet Mohammad struck a legendary, ten-year hudna with the Quraysh tribe that controlled Mecca in the seventh century. Over the following two years, Mohammad rearmed and took advantage of a minor Quraysh infraction to break the hudna and launch the full conquest of Mecca, the holiest city in Islam."
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Maybe someday we in the West will catch on to what moslems really mean by hudna.
In other words, he practiced peace when he was weak and war when he was strong. It is noteworthy that the verses of the Koran writen in the "peaceful time" speak of living in peace with other religions, after he took Mecca, the verses speak of the destruction of all infidels. And you would think we could understand them. Its all out there to read.