BELGRADE, Serbia (AP) — For years, Russia has worked to gain influence in Southeast Europe, using Serbia as a foothold to establish a friendly pocket on a hostile continent.

The European Union finally is pushing back. European Commission President Jean-Claude Juncker is embarking on a seven-nation Balkans tour Sunday to promote the EU's new eastward expansion strategy.

Russia mainly wants to discourage the Western Balkan countries — Albania, Bosnia, Macedonia, Montenegro, Kosovo and Serbia — from joining NATO. But Moscow also is trying to deter them from joining the EU.

The EU sees the prospec...