William Hague has vowed to base foreign policy more clearly on the UK national interest if the Tories win power.
The shadow foreign secretary said he rejected claims the UK's influence in the world was shrinking and could only be exerted through the European Union.
The Tories would never commit troops abroad without a "full explanation" of their mission, he told its conference.
The party said it would also create a homeland security force to protect the UK from terrorist attacks.
Reports have suggested the permanent force may consist of 2,000 troops and be designed to deal with potential city-centre attacks similar to those seen in Mumbai last year.
Security spokeswoman Dame Pauline Neville-Jones said Labour had left the UK "unnecessarily vulnerable" to attacks on its soil and a "dedicated" new unit was needed to enhance UK security.
On Afghanistan, Mr Hague said that while the Tories were fully committed to the current mission, they were equally clear that British troops could not remain there indefinitely.
We should never be ashamed to say we will promote our own national interest
The UK's objectives in Afghanistan were "clear", he said - to ensure Afghans could "provide for their own security and livelihood without presenting a danger to the rest of the world".
On Europe, Mr Hague reiterated the party's commitment to a referendum on the Lisbon Treaty - the subject of repeated controversy during the conference - saying its centralisation of powers was "against the spirit of the age".
However, as it emerged that Poland would ratify the Treaty on Sunday - leaving the Czech Republic as the only remaining EU member not to do so - he said nothing about what the Tories would do about a referendum if the treaty had been ratified.
More broadly, he promised a "distinctive" foreign policy based on an "enlightened" British national interest.
Under the Tories, the UK would promote free trade and sound foreign aid and play a supportive role in international efforts to resolve long-running conflicts, to prevent nuclear proliferation and tackle climate change.
"We should never be ashamed to say we will promote our own national interest for the British national interest is no narrow agenda." he said.
Under Gordon Brown, the UK's influence in the world had "diminished" while the government had "mismanaged" its relationship with the US.
He promised to strengthen the US alliance, develop bilateral relationships with other leading powers and create a National Security Council to co-ordinate defence and foreign policies.
"We reject to be agents for the management of our country's decline," he added.
Labour said the Tories were "skirting" the real issues surrounding Europe and the public "deserved a clear answer" from them on their intentions over Lisbon.
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