Then came Hitler's disastrous attack on the Soviet Union.
Given what had happened in Russia under Stalin in the 1930s, that hardly seemed deserved.
As Mr Brown notes, Stalin trusted the Nazi leader more than he trusted his own generals.
But his main expertise, acquired over decades of scholarly study, is in the Soviet Union and its east European empire.
His account is studded with delightfully pertinent and pithy personal observations and anecdotes: the censors in tsarist Russia decided that Karl Marx's “Das Kapital” was so boring that it wasn't worth banning.
In other countries, such as Poland and the Baltic states, it looked different: one occupation gave way to another.
The promised communist nirvana brought a mixture of mass murder, lies and latterly the grey reality of self-interested rule by authoritarian bureaucrats. Communist regimes proved remarkably durable, partly thanks to the use of privileges for the docile and intimidation of the independent-minded.
During the mid-1940 's the United States and Russia were in superpower positions after World War II had ended and left Europe in a pile of rubble.
Continuous tension between the United States and Russia led to the beginning of the Cold War.
Readers over the age of 40 will find it an uncomfortable reminder of a dangerous and dismal past.
For most younger readers, it will seem all but unimaginable.