We construct a solution to the low-energy string equations of motion i
n five dimensions that describes a circular loop of fundamental string
exponentially expanding in a background electric H field. Euclideaniz
ing this gives an instanton for the creation of a loop of fundamental
string in a background H field, and we calculate the rate of nucleatio
n. Solutions describing magnetically charged strings and p-branes, whe
re the gauge field comes from Kaluza-Klein reduction on a circle, are
also constructed. It is known that a magnetic flux tube in four (reduc
ed) spacetime dimensions is unstable to the pair creation of Kaluza-Kl
ein monopoles. We show that in (4 + p) dimensions, magnetic (p + 1) ''
flux-branes'' are unstable to the nucleation of a magnetically char ed
spherical p-brane. In ten dimensions the instanton describes the nucl
eation of a Ramond-Ramond magnetically charged six-brane in type IIA s
tring theory. We also find static solutions describing spherical charg
ed p-branes or fundamental strings held in unstable equilibrium in app
ropriate background fields. Instabilities of intersecting magnetic flu
x-branes are also discussed.